In your fs2002/Aircraft directory, find the folder for your 737-500. Inside that folder you'll find the Aircraft.cfg file, which can be opened in Notepad.
Scroll down and you'll find something that looks like this:
[fuel]
Center1=0, 0, 0, 2313, 0
LeftMain=-3, -19, 0, 1500, 0
RightMain=-3, 19, 0, 1500, 0
For each tank, the first three figures refer to its dimensions, and the fourth refers to its capacity in gallons. This can be edited.
Be aware that increasing the fuel capacity significantly will also increase your takeoff weight significantly. The .air file (also in the aircraft folder) will cause the aircraft to react accordingly. Therefore you will want to choose the longest runway available for departure. Also make sure your fuel load is balanced!
Good luck!
