Can we change automatic car gear while driving like D to L2 ?is it safe?

can you yes. but why just leave it alone

"Can we change automatic car gear while driving like D to L2 ?"

Who is this "we" you speak of?

"is it safe?"

Generally speaking; it's safe. All you're doing is limiting the transmission to 1st and 2nd gears, and forcing a downshift. Most cars made in the last 25 years or so won't allow the transmission to downshift into too low a gear that would result in the engine speed exceeding redline. With that said, there's only a few situations where it would be considered advantageous to do so.

You should not go above certain speeds in the lower gears. Electronically controlled transmissions might not even downshift while you'e going the wrong speed for it.

Yes, it changes the gear ratio in your transition so you'll have higher RPM at a lower speed, creating more torque.

yes, not a problem, going down hills i drop down so i don,t have to use the brake