Deleting Files

For files see thishttps://support.apple.com/en-us/HT204055

Then drag and drop the files from the location in the finder to an external hard drive see the article https://support.apple.com/en-in/guide/mac-help/mchl027f1d66/mac

Why not just copy all your files to the external drive for safety? Then set up your new computer, and let iCloud sync the files there to it. Later, you could remove the old computer from syncing with iCloud if you want.