-To get it, use the Network update function if your PSP is on 3.80 M33 Firmware. It will download the new firmware automatically.
-However, the network update does not include the official 3.90 firmware, a file required for the update. (I suggest just updating like usual; network update good for only minor updates from M33. Please verify this, I am not sure because I already downloaded the update)
-If you do not have 3.80 M33 Firmware or no wireless connection, download from the below link and install as usual.

Download
http://rapidshare.com/files/115348008/PSP_390_M33_UPDATE_FL.rar 

3.90 M33 Firmware Changelog/Readme: 

3.90 M33
--------

Requeriments:

- Installation requires a PSP with 3.52 M33-3 or higher.

Instructions

- Copy the UPDATE folder into /PSP/GAME/
- Get the 3.80 sony update from somewhere, and put it in same folder, with name "390.PBP".
Alternatively, you can let the installer to download for you via wifi.
- Run it, the update will be done by Sony updater. At the end when you are asked to reboot the
PSP pressing X or O, do it.


Changes:

- 3.80 -> 3.90
- March33 NO UMD: fixed (yet another) bug related with exiting with home. (infinite semaphore wait)


Changes in updater:

- The degeneration check and correction of IDS keys happens too in 3.71 now.
- Added code to download 3.90 PBP from net via wifi.
Note that his code is not yet 100% stable, it may crash when selecting AP. In that case,
reinit the installer.
- L+triangle has been restored as method to bypass battery.
If you already have 390.PBP keep those buttons pressed until you see "Veryfing 390.PBP".
If the 390.PBP is being downloaded bia wifi, press those buttons at the end of download until
you see the previous sentence.