Change to passive 1

Sometimes it is not suitable to use passive.
From the sentences you have given , i can see there is no need to change.
Having these sentences being passive voice, the meaning of sentences would become abnormal.

If you really want to have the answer, see below.
1. The examination is finished.
3. A new computer is bought by Ms Chen.
4. A soccer is being played by the children in the park

The students was finished the exam.
We were happy because the spring had came.
Ms chen had bought a new computer./A new computer had bought by Ms chen.
The children were played soccer in the park.
It was rained hard last week .

The exam was finished by the students.
除非更改句子的結構，否則此句不可以轉為被動式(We are made happy by the coming of the spring.)
A new computer has been bought by Ms chen.
Soccer is played by the children in the park.
It rained hard last week = 此句不可以轉為被動式

Yes joyceyychan's opinion is right.
But there is still room for improvement in the answer provided.

The 2nd and 5th lines are fine there is no need to change.
The rest we will need to take a look at the tenses involved.

1. The exam was finished by the students. / The exam was finished.

3. A new computer has been bought by Ms Chen.

4. Soccer is being played by the children in the park.