how could france and britain give the sudetenland to germany when it was part of czechoslovakia?

Well, it had only been part of Czechoslovakia for nineteen years. Come to that, Czechoslovakia had only existed for nineteen years.

The idea that national boundaries are fixed and immutable simply didn't exist back then. The boundaries of Europe had been re-drawn at least once a generation for a thousand years - and people often found themselves compatriots of those with whom they had no affinity.

Briefly, the Austro-Hungarian Empire was broken up by the Treaty of Versailles, leading to the creation of Austria, Hungary, Czechoslovakia, Yugoslavia - and land being granted to Poland (re-created in 1919 - in 1914, Warsaw was a Russian city) and Rumania. Note that Yugoslavia disintegrated messily in the 1990's, and Czechoslovakia split itself in two (peacefully).

The Sudetenland was home to ethnic German-speakers who found themselves on the wrong side of a border they hadn't chosen. Most of them WANTED to be part of Germany or Austria (though after the Anchluss, this amounted to the same thing), rather than an invented country who looked down on them. Most of them, but not all - there was a lot of ethnic tension. This is what Hitler sought to exploit.

As for Britain and France - their principal concern was to avoid another world war. Yes, things didn't go as they'd hoped, but the best-laid plans of mice and men...

Mind you, short of starting WW2 a year early, it's hard to see what they could've done. Britain in particular had known for three hundred years that geography made them incapable to getting involved in a European land war without a major ally. France, on whose soil the Western Front of WW1 had largely been fought, was desperate to avoid a repeat.

In short, Britain and France gambled that the Munich Agreement would avoid WW2. The were proved wrong, but you can't really blame them for trying.

Because at the time the German military had the Sudetenland surrounded on the 3 sides and Germany was 15 times stronger than Czechoslovakia. There was no way the Czechs could hold on to it unless Britain/France fought for them and Britain/France found it easier to kiss German butt and betray the Czechs than dare fight Hitler who they were afraid of. 
Britain/ France telling Hitler they would not fight for the Czechs and that he could have Sudetenland is all it took for Hitler as the Czech's cut off with no allies surrounded on 3 sides and outnumbered 15 to 1 didn't even have to be consulted.    

They had little option to sell Czechoslovakia down the river. But it would be one of the actions that would make them ally with Poland. But let's have a reality check: they had no way of stopping Hitler at that time.

The Munich Agreement was an agreement concluded at Munich on 30 September 1938, by Nazi Germany, the United Kingdom, the French Third Republic, and the Kingdom of Italy. It provided "cession to Germany of the Sudeten German territory" of Czechoslovakia. Most of Europe celebrated the agreement, because it prevented the war threatened by Adolf Hitler by allowing Nazi Germany's annexation of the Sudetenland, a region of western Czechoslovakia inhabited by more than 3 million people, mainly German speakers. Hitler announced it was his last territorial claim in Europe, and the choice seemed to be between war and appeasement.

They didn't. They merely agreed to not interfere when Hitler took it under threat of force.

It was appeasement 19 years before millions of men lost their lives in the butchery of WW1  and Europe were still mourning sons brothers dads it was extraordinary to think of risking that again . They did not give it they accepted his demands as long as this was all he wanted to save the millions of lives that were eventually lost in WW2 29 million souls Russia lost in WW2 so it was worth at least a try if nothing else as they say the rest was history Hitler lied 

Back about 1820 as a development of the Napoleonic wars and the romantic movement, the popular mind began believing in the formula one people/one language/one state. This was an improvement at least over the old one god/one religion/one people formula, but it meant that all over Europe everyone who spoke Italian or Polish or Croatian now believed all such populations had to be united, even if Croats (for example) were only a minority of X state. It would be one thing if this was about literature and folk songs and cuisine, but it had to be achieved violently, because that's the path to national glory. All the political turmoil in the Austro-Hungarian Empire prior to WWI was exactly about this issue, with even small peoples screaming for independence (national self determination is one of the worms that escaped from the American Revolution).
National self determination is a strong ideal, but in central Europe, where populations of Poles and Czechs and Germans had been shifting for hundreds of years, it was a very serious problem, greatly aggravated by Nazi claims of German racial superiority. There was considerable friction between the Sudeten Germans and the Czechs (including bloodshed just after WWI). Czechoslovakia itself was created by the Versaille Treaties. What France and the UK granted, they could take away - so it seemed.

They didn't GIVE  it to them dumdum, they didn't oppose the German annexation of it militarily. Big difference.