Just launch Lytro Desktop, and any available update will be offered to you automatically. If Desktop is already running, just choose Lytro > Check for Updates (Mac) or File > Check for Update (Windows). In either case, if a new version is available you'll see something like:
IMPORTANT: Sometimes it may take a day or two before you are automatically offered a new update. To update immediately, from the top menu in Lytro Desktop select:
Lytro > "Check for Update..." (Mac)
File > "Check for Update..." (Windows)
NOTES:
To see the current Lytro Desktop version on your computer: