I'm about to reinstall my windows7 and i need to know something?

So i need to reinstall my laptop's windows 7 home premium 32 bit and i'm about to change it to ultima 32 bit and i already have a USB with 4gb and i found the windows and instructions but i think that the instructions are on installing on pc who does not have a windows which means install and what i'm trying to do is get rid of my old one and get the new one so if i just follow the install steps would it work and the old one would be overwritten or i first need to do something to uninstall the old one then install then new one PLEASE help i need to start in like 2h