On the Vita, select > Start > System Update > Update by Connecting to a PS3 system . Manual USB Update (PS TV Only):
The search for a is a journey toward stability, preservation, or hacking. Whether you use the official Sony CDN link provided above or a community mirror, ensure you understand the implications of updating. ps vita 374 firmware link
If you have the PUP file from the link above, use this method. It is essential if your Vita is currently on a hackable firmware (3.60) and you want to preserve certain exploits. On the Vita, select > Start > System