Can I replace my existing free .rf.gd domain with a registered domain? Like if I want to replace lolywood.ref.gd with lolywood.com, can I do that?
Replacing a domain is very easy. Just add the new domain to your account in the Addon Domains section, from where you can also delete your old (sub)domain.
@Ataulhaq like @Admin said add your new domain throught Add-On Domains section. Make sure that you have registered it first and that you have pointed it to InfinityFree’s nameservers. After that, simply add it, and you will find a folder with your domain name in File Manager, and open it and that folder will have htdocs folder, which will be the folder where you upload your public files. Then just delete the old subdomain. 
Thank You.
Thank You Admin.
Thank You moderator.