Sorry for the late reply, it's seems like it's time for your Belta to flush/replace it's CVT fluid.
Yes, you have to open the drain bolt and drain the fluid completely. Let the fluid drain out throughly for around 10 minutes after opening the drain bolt. Yes, you will have to replace the drain bolt washer aswell.
Correct, after the CVT fluid has been throughly flushed it's time for you to pour in the new CVT fluid, it mostly takes 4 liters. When your pouring the CVT fluid pour it in slowly and calmly don't rush. When the CVT fluid is almost at the top stop pouring it. Don't over-pour it just top it up.
After pouring the CVT fluid and closing the filler bolt properly you will have to let the engine run for 10 minutes, shift the gears aswell. Check for leakages all around the gearbox and see if the CVT fluid isn't leaking through the filler bolt. If it isn't and your gears are shifting smoothly take your Belta for a ride around the street and check if the gears are still jerking or not.