If it is just code that needs to be updated then: 1. Backup your code and db 2. Delete old code 3. Put new code where you need it 4. Adjust any configuration files as needed
If you you want to point your domain somewhere else then you have to do that in the DNS management. Before you point the DNS somewhere else and you have a SSL cert. check up on what you have to do depending on the type such as domain validation etc. If you are using APIs like google analytics, check up on any domain validation stuff you may have to do.