How can I restore from a older backup?

How old that "older" backup?

When you restore from a backup, you must first erase all the data on the phone and then start over again with a new setup from the "hello" screen. If you use the "older" backup to restore the phone, your phone will look like it did on the date of the backup.

Any new data that you have added to your phone since the date of the backup will be lost, so if the "old" backup is 3 months old for example, there will likely be a huge loss of data on your phone.

If you backup with iCloud, you should have access to your two most recent backups. See the response by picas in this discussion.

Restoring of a DELETED iCloud backup of a… - Apple Community

If you backup on a computer, it is possible to have multiple backups if you archived your backups. This is very easy to do on a Mac but a bit more convoluted in Windows. In days past, I typically had three older backups for each of my iOS devices in iTunes on my Mac. I quit backing up with iTunes a while ago but the backups are still there.