[Unlocked Bootloader] Flash from 5.0.2 to 5.1

Hello community,

I wanted to download the new lollipop update, but the OTA fails. The reason is: I unlocked the bootloader (without root).
What is the fastest way to update to 5.1? Is there any way, keeping my data?

If not, what is the best way to backup these data?

Thanks for your response!