You can go about removing the salt in two ways -
Remove him and place him in the cup you got him from, siphon all of the water out of the tank (don't change the filter media) and then replace the water. Float him in the cup in his tank, every 5-10 minutes remove about 25% of the water from the cup (dump in the bowl or sink, use the lid to make sure he doesn't jump out) and replace what you took out with water from the tank. Do that about 4 - 6 times. Then use a net and gently pour him into the net (over a bowl) and then gently place him in the tank. That way, with the removal of old water and added new water he will adjust to fresh water.
Or you can do a few 50% water changes every day for a few days to help lower the amount of salt in the tank.. will take longer to remove it, but it's an option.
There are a lot of people who claim that using the salt is the best thing, but those people may not understand how a body works, and how diseases/parasites become immune to medications when over exposed - it happens with humans as well. It is still out there in practice because people will see one person do it, so they do it and they swear by it, and it gets passed around. Doesn't make it right, but it's out there. Sometimes you have to take what you read about betta care with a grain of salt.. pun intended :)