My computer says I don't have enough space to download Big Sur. How to I correct that?

First, how much free space to your have? Right-click your hard drive icon on the desktop and choose "Get Info" from the resulting contextual menu. That action gives this:

On your computer, what do those three lines report.

Big files are usually music, images, and movies.

To safely move images in the Photos app to an external drive, see this:

Move your Photos library to save space on your Mac - Apple Support

ujessica wrote:

My computer says I don't have enough space to download Big Sur. How to I correct that?

Big Sur states that you will need 35.5 GB of free storage when upgrading from High Sierra or newer, and 44.5 GB when upgrading from an older versions.

Empty your Trash — Delete files and folders on Mac - Apple Support

https://support.apple.com/guide/mac-help/delete-files-and-folders-on-mac-mchlp1093/mac

How to free up storage space on your Mac - Apple Support

https://support.apple.com/en-us/HT206996

See used and available storage space on your Mac

https://support.apple.com/guide/system-information/see-available-storage-space-syspf9b375b9/10.14/mac/10.15

User tip: "Other and What Can I Do About It ?"

https://discussions.apple.com/docs/DOC-5142

Try something like OmniDiskSweeper for a GUI to get a good look at itemized file size and location:

OmniDiskSweeper http://www.omnigroup.com/more