How do you put up a fan with lights up where there were just lights?

I am assuming you mean a ceiling fan with a light kit attached. You need to make sure there is a box that you can attach the fan too. I know that sounds stupid, but we live in an old house, and found lots of plain wires with no box. I replaced a ceiling fan myself, and it was really simple. Here is a link to a tutorial that may help https://www.diynetwork.com/how-to/skills-and-know-how/electrical-and-wiring/how-to-replace-a-light-fixture-with-a-ceiling-fan

You will need to determine if electrical box in ceiling is full size (not pancake) and firmly attached to a ceiling joist to support weight of fan. If not buy a stabilizer bar which you can fit first.

See link attached for one retro bar available. Good luck! https://www.homedepot.com/p/RACO-RETRO-BRACE-with-4-in-Round-Ceiling-Rated-Pan-2-1-8-in-Deep-with-1-2-in-KO-s-937/203743172?cm_mmc=Shopping%7CVF%7CG%7C0%7CG-VF-PLA%7C&gclid=Cj0KCQjwnqzWBRC_ARIsABSMVTNZSTuyF1_gspswCj29sMyct1jabAG2XSQFTicYhV-q_AvJnDBHXaAaAlxrEALw_wcB&dclid=CJ6647mwrdoCFZVCDAodGpsP6w

The difficulty with swapping a light for a fan is not in the wiring, but in the electrical box. A fan box should be used to support the added weight of the fan. This may be easiest if there is access from an attic crawl space. If not, removal of the ceiling sheetrock may be necessary to install the fan box, then repair and paint the sheetrock.

The electrical connections are the same. Your fan will come with wiring instructions, but it's very basic. The most difficult part is the support. A fan is heavier and must be level. There are drywall support brackets that go up into the ceiling after you remove the light. Usually you need to buys these separately from the fan. Instructions are also included with the bracket.