@Flicknetcher
Do a full backup to itunes or icloud then factory reset the 11.
This will give you enough space to update the 11 and reinstall the backup to the it.
You can then do a phone to phone transfer or install the backup to the 15.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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