No way to make enough space for Big Sur

The bottom line is that you have two options;

1. Delete old files off the internal drive to make room.
2. Buy an external drive.

If your Mac is Pre-2013 then you can buy another internal drive.

You can also use the external drive to copy old files to, if you don't want to actually delete them.

How much free space do you have.

Quite often, or more than often the Disk Usage read out in About This Mac Storage

is incorrect, this can sometimes be rectified by reindexing Spotlight.

How to rebuild the Spotlight index on your Mac – Apple Support

Move the Macintosh HD (or the name you gave your disk) in to the Privacy panel.

Quit System Preferences.

Open System Preferences> Spotlight> Privacy highlight Macintosh HD and press the minus button.

The mac will start rebuilding the Spotlight index.

Check by clicking on Spotlight in the menubar and enter word, if it is indexing you should see a progress bar.

If re-indexing has not solved your problem then run this app, https://www.omnigroup.com/more

this will give an accurate account of the storage used. When the app has created its overview you can look at

the Users folder in the output and see what each user has stored, you can then delete files from there.

Do not delete any files or folders in any System or Library folders or any files you do not understand.

Did you already delete all the deleted items? Also for mail?

And maybe you can delete some apps like Garageband?

Or do you have an usb where you can store some of your files/photos/movies temporarily ?

What does "storage" say? You need around 45 gb to update right? And system use around 15 gb? Maybe we can find where all your storage goes to. You have already deleted all the former execute files/download files for former updates?