How to free up space on mac for updates

I understand you are unable to upgrade macOS due to a lack of available storage. Is that correct?

First please review Free up storage space - Apple Support.

If you are still unable to create sufficient space for the upgrade after exhausting all those steps, the workaround is to create a Time Machine backup, completely erase the Mac, install the latest version of macOS (read • below), and finally transfer the contents from that Time Machine backup when asked. To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support.

• To install the latest macOS that is compatible with your Mac, refer to If you're using any other Mac in How to reinstall macOS - Apple Support.

if you are running low on space, you can purchase an external HDD or SSD of sufficient capacity and move your larger files (videos, pictures, documents, etc...) to it. once they are on the EHD, you can simply erase the same files from your internal drive. and, if it's just going to be used for storage, you can use an HDD and save a few bucks over the cost of an SSD. FWIW, i personally keep very little on my internal drive.

additionally, you want to keep ~ 20% of your local drive as free space for proper operation of macOS.