Webflow templates aren’t themes, they are a foundation for building your site.
So the cleanest way is to-
Rebuild your site using the new template project
Transfer your hosting plan over to the new site
It’s also possible to effectively “merge” the template into your existing site, piecemeal, but it requires a solid understanding of classes, draft, visibility: hidden, and how to work in a production site. You’re basically rebuilding the plane while it’s flying.