Hopefully it is just low on fluid. Check that and top it off if low.
Otherwise it will need to be serviced by a trans shop. You can prevent it from shifting by moving the lever to a lower gear. I can't remember exactly what that lever looks like but try that. Don't drive it fast in 2 if all you have is 1 , 2, and D. That could ruin the engine.
Answers & Comments
Hopefully it is just low on fluid. Check that and top it off if low.
Otherwise it will need to be serviced by a trans shop.
You can prevent it from shifting by moving the lever to a lower gear. I can't remember exactly what that lever looks like but try that. Don't drive it fast in 2 if all you have is 1 , 2, and D. That could ruin the engine.