How to get old backup

You can't......unless you specifically and manually "archived" and dated different backups.

Otherwise, each backup simply overwrites new data over what is present on the backup, so in effect you only have one current backup. I think iCloud may store a few recent backups.

Archive or make copies of the information you store in iCloud - Apple Support

Now.....if you have been backing up your iPhone to your Mac......AND.....you have been using Time Machine to back up your Mac to another hard drive, then the complete history of backups is stored in the Time Machine backups and you should be able to go back and pick up a backup from a specific date. As you might imagine, things can get complicated.

You can’t; a computer backup is only the most recent backup, and an iCloud backup keeps only the 2 most recent backups, unless you back up to a Mac and chose to archive an earlier backup.