iPad update 16.7.13 won't download

Available information indicates that the iPadOS 16.7.13 update has been [temporarily] withdrawn by Apple. If already downloaded the existing download will not install. The associated digital signatures, that must be successfully verified immediately prior to installation, have been revoked by Apple.

At this time you would be best advised to save your efforts in attempting to update your iPad. Hopefully the issue will be resolved by Apple within the coming days.