That really depends. Do you suspect it has a blockage, parasites, egg bound or bacterial infection? All have different treatments.
If it's just backed up you can feed it thawed frozen peas. If the fish is bigger you can even put an epsom salt in the pea to help relieve the fish. Watching it closely to be sure it poops.
You can also do epsom salt dips. Using tank water in a separate bucket add 1tbls per gal and put the fish in it for 15-30mins. Watching to be sure it doesn't get too stressed. And again watching to make sure it poops.
The poop will also give you a better idea of what's going on with the fish.