Why the update to Android 5.1 fails?

[AC Question]

Several times a day, I receive an invitation to update. It comes back until I accept. When I accept, the installation fails. I cleaned the cache and it did not help. I do not want to reset the factory settings, for now

 

[B. Diddy]

Welcome to Android Central! Which phone? Does it fully download, and then the installation fails? Or does it fail during the download process?