Updating backup

Any error messages given?

Are you trying to create a backup on your computer or in iCloud?

How to back up your iPhone, iPad, and iPod touch with your Mac - Apple Support

How to back up your iPhone, iPad, or iPod touch with iTunes on your PC - Apple Support

How to back up your iPhone or iPad with iCloud - Apple Support

You will need enough free space in iCloud and/or your computer during the backup process. At least 2-3x the size of the final backup data.

And, if you are creating a backup to iCloud, your device as to be charged during that time, to keep the Wi-Fi connection.

If it enters standby mode without being charged, the Wi-Fi connection will be switched off.

Do you see any errors when you try? Assuming that you are attempting backup to iCloud, a Windows PC (using iTunes for Windows) or Mac computer (using iTunes or Finder - as appropriate for the installed version of macOS), these support pages should help with troubleshooting:

If you can't back up to iCloud - Apple Support

If you can't back up your iOS or iPadOS device to a computer or restore it from a backup - Apple Support