How to upgrade an already dound spectre gem?

I bound something into a spectre gem as soon as i got my first lvl 7 gem. Now that i have higher level gems i wonder how to upgrade it.

Upgrading other gems works by just creating the same skill while you have it equipped. You see a small + sign on the skill icon indicating that. But that is not the case for the spectre gem as the equipped one is the bound version.

So how do i upgrade that?