Yes.You can trade your fuel module in outfitting (at a station WITH outfitting, look in the information panel, in the system map to check that) .. and switch it back, by buying a cargo rack. However, if you have left the station where you bought the fuel module, you may lose money on it (depreciation).
Safe to say we've all done such things, and the manual is tricky, mostly because you have to read it start to finish and can't search it .. the elite dangerous wiki is good for orienting yourself to the way the station services work (
link) .. ED does have a steep learn curve but you'll get there, no worries.