How to replace a bathroom fan?

Hi Beth, perhaps this DIY with help. https://www.azdiyguy.com/blog//2013/04/replacing-our-bathroom-exhaust-fan.html

pick out a fan about the same size as you currently have. instructions for installing should be with it. kill the power to the room. have a friend read the instructions , as you implement them. most of the time its 3 wirers, black, white or red, and ground.

Why are you replacing the fan? Broken or not doing it's job? If it's not doing it's job, you may need a new or a bigger one. When you buy the new one, make sure that it's for your size bathroom. As well as doing what Dfm said, make sure to connect the duct to the fan so the moisture gets blown outside.