Change group from Private to public

Hi,
I set up a group as private but need it to be public for a while, then will want to change back.

Is there a way to do this?

I've been trying to upload the cover image.

I've cropped it and resized it a couple times, but when I drop it in it always 'expands' my image and chops off the top edge.

Is there a certain size I should use? Or ...?

Jojo tried to help but couldn't get in:
Jojo-Schmitz
Aug 22, 2020
"This group is unavailable"

Here's the page:
https://musescore.com/groups/garage-band-theory

Thanks,
DS