Hi I recently installed iOS 26 but did not make a backup, how can I get iOS 18 without losing any data?
iOS 26 restore
Have a look here: https://discussions.apple.com/thread/256078060?sortBy=rank
and here: https://support.apple.com/en-us/108763
But I would be cautious without a backup.
So, you may also wait (a couple of months) for an iOS 26 release and upgrade at that time. Your data should be safe.
Then, don't forget to make a backup.