How much would it cost to swap transmitions on a bike?

Bikes and bike components are very specialized. As it sounds like you have little experience in the fixing and putting together of bikes, I would not attempt to switch anything out at this point. What is wrong with having only 7 gears? It’s not like you’re going to be racing or doing any hard commuting on a folding bike anyway, so 7 gears will likely be fine for you.

I guess the important question is this: What will you be using the bike for? I guess if I had more information from you I could answer your question better.

I’The bike, all the bikes, I had in mind are 26 inch wheels. I’d be using it for somewhat long distance riding, maybe 30–60 miles a day. I do not now anything about bike assembly. I’m located in central Oklahoma and I currently ride a 21 speed bike. I’m afraid that with only seven gears I won’t be able to handle the steeper hills

Well, if you want to ride that much, getting a bike with 26” wheels is a mistake. You will be exerting much more energy than you would with a 700C wheeled bike. I would recommend getting a road bike with narrower tires (700 X 23). If you know what size you need, you may even want to check out your local craisglist ads for a used bike. You will be seriously regretting using a folding bike with 26” wheels to go more than 10 miles on non-flat surfaces. What is your price range? Maybe I can help find something for you.

around six hundred dollars

Hmmm…for $600 I would look on craigslist and ebay. You’re not going to find anything new (decent) for $600. Make sure you know what size bike you need before buying anything. I’m 6’2” with a 34” inseam, so I ride anything from a 58cm to a 60cm road bike. You’re going to want around an inch of clearance between the top tube of the bike and your body when you are standing over it. It would be worthwhile to take a look at some bikes at your local bike shop to get a feel for what you want and what might fit you well.

also consider raising your price range if you ride that much : ) I’m jealous you get so much time on the bike! I send brownlemur that you should get a road bike, not a folding one.