If I have an iPhone 5, how do I get a new iPhone the next level up?

I don't know anything about this stuff but I have an iPhone 5 and I've read about trade ins and stuff like that. How could I… If I can… Get a new phone? In place of the 5?

You need to talk to your iPhone company provider. For example, mine is T-Mobile. You can get a phone warranty on your iPhone so when the next generation of iPhone comes up, you wouldn't need to pay extra amounts of money. I put a warranty on my iPhone 6s and when the iPhone 7 came out, I switched to the iPhone 7 without paying the entire amount of the iPhone. The warranty is around $30/mo plus the iPhone. It is best to talk to your iPhone provider. Hope this helps.

Trade it in at stores/sites that offer that
Sell it, then buy the new one or vice versa

Nothing difficult about that.