I have a John Deere L120 that has about 400 hours of use. When I use it now it will not pull up hills when it gets hot. Can I change the fluid in the transmission or should I just junk it? If it can be changed, how much fluid and what type?

That was common for those models , most only made it about 250-300 hours.there was a recall on the tranny but it was a fix as fail type thing.
The transmission is about 400 - 600 dollars depending on were you get it. If you get the number off the tranny , and look on Husqvarna you can order one there , it a peerless transmission and Husqvarna bought them, if you get at Deere you will be paying for green paint . Luck