Just click the dots on the alert and hit cancel...
It'll delete the downloaded file. It's what I've been doing.
I guess the update file is loaded in this folder: ud0:/PSP2UPDATE/PSP2UPDAT.PUP, I opened this file through a hex editor and I found "3.61" string were specified in several places. So if you have this file - delete it. My file weighted about 12Mb so Vita didn't manage to finish download. I don't know it resumes download or not but if it does - you could wake up with an update one day.
As far as I'm aware that would require editing, repacking and resigning the update file to be able to even be installed. Unfortunately, we're not in the PS3 hacking era anymore where downgrader firmwares can be made for the higher, more recent firmwares of today. And Sony have learnt well from their mistakes, so making update file modifications like those are going to be extremely difficult (if not, impossible anytime soon).I wonder if the 3.60 update can be spoofed to 3.62, so it will overwrite 3.60?