I can't update mac software

What error are you getting? Without knowing that, I can only assume it's based on lack of space.

Freeing up space: 

Free up storage space on your Mac - Apple Support. 

Optimizing Space:

Optimize storage space on your Mac - Apple Support also offers some suggestions.

Another thing you can try is to try to update while in safemode:

Start up your Mac in safe mode - Apple Support

Hi

1. you have mac which dosent get support to have newest version of mac os
2. you have full storage