After I've backed up my MacBook Pro, what do I need to delete in order to have more ROM so I can download the new OS?

A user has no control over “Other” category of storage.

• Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

       https://support.apple.com/guide/system-information/see-available-storage-space-syspf9b375b9/11.0/mac/11

What a user can do?

   1.  Delete files

       https://support.apple.com/guide/system-information/find-and-delete-files-syspf5a64aa6/11.0/mac/11.0

  2.  Optimize storage

       Click the “Manage”button. Wait until Green “Completed” button appears.

       https://support.apple.com/guide/system-information/optimize-storage-space-sysp4ee93ca4/11.0/mac/11.0 

 3.  How to free up storage space on your Mac

      Try all steps posted in this article

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

dominic23 wrote:

A user has no control over “Other” category of storage.

Other: • Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

       https://support.apple.com/guide/system-information/see-available-storage-space-syspf9b375b9/11.0/mac/11

"Other" relates primarily to data stored inside applications (usually third-party apps), and usually lives inside one or both of these locations:

/Library/

~/Library/ (~ denotes the location of your user account folder)

To manage the "Other" category, please see this User Tip: What is “Other” storage on a Mac, and how… - Apple Community