If I understand it correctly: you have already built a website on our hosting. You say that “the domain is with IF”. But we’re not a domain name provider, so that’s not possible. Or do you mean that your website is setup on a free subdomain?
If so, the first step is to add the domain to your InfinityFree hosting account. Be sure to set it up as a Parked Domain (if using the control panel) or set the domain to be linked to the same directory as your subdomain (if using the client area).
After that is done, you may need to change some additional things in your website to start using the new domain. The exact steps for this depend on the website software you use, so you may want to search the web for that.