High Sierra Has Switched My Administrator Account To Standard?!

Can anyone help me?

I recently tried to change my username & in the process, which I didn't realise until now was that in doing so it has switched my administrator account to standard. Now I can't make any changes, nor download any software.

I own a MacBook Pro (13-inch, Mid 2012) running MacOS High Sierra 10.13.3

I've updated the software for it along with the supplemental one thinking that it would fix it & it hasn't.

Does anyone know how to fix this?

I'm no tech expert & don't want to fiddle about with it too much & make things worse.

Any help would be appreciated.

Cheers 😟