Hey @Borislav.VD,
After license conversion (instruction given in the previous reply), you need to download Pro in your dashboard. You should be able to see this:

If you’re going to use a child theme, you can download Pro child theme at https://theme.co/apex/child-themes
Next, install Pro and activate it if you’re not going to use a child theme. Otherwise, after Pro parent installation, install and activate the Pro child theme and that should be it.
Regarding the X theme license issue, that is weird along with X theme that has header and footer in the menu. Please delete X theme in your install. Also, try revoking the license and register your site with Pro license.
Thanks.