Repairs STX38 Hydrostatic Drive

My John Deere STX 38 (black deck) hydrostatic drive works fine when I first start the mower but after it warms up it loses it pulling power really noticeable when you shift into reverse or go up hill.

I have no manual it came with the house when I bought it and have gotten 4 good years out of it.

How do check the fluid level? Or change the fluid? Is there an adjustment I can make?

Thanks

John