How to I edit premade buildings that I download from asset store?

I have downloaded a medieval asset that gives me lots of buildings but how can I edit them to my liking?

You can edit 3D buildings using a modelling program. A popular tool is 3DS Max. It’s a relatively expensive tool (compared to other applications) however if you are a student, you can get it for free.

Some 3D models can be edited with Blender (which is free)

There are lots of other 3D modelling programs. You need to look at the file ending to see what kind of file it is, then use a program that supports it. Ideally, you learn the tool first, then find files (or acceptable conversions) with which you feel comfortable