If your domain is already successfully registered in the SiteGround provider system, and you want to manage its domain names, you will need to open Services - Domains - Manage. Once you go into these settings, you will find a link to the Manage Nameservers page next to the domain you are interested in. Then it remains only to fill in all the necessary information and replace the old information with the new actual DNS data of the new server. That is not difficult, but if you have any problems — write to support.