well, regular network update to me means that you download and run an eboot, be it via wifi or from a site.
you can download the m33-6 update you need here:
Custom Firmware 5.00 M33-6 Released, PSP Hacks - PSP Slim Hacks
just download, unpack, move the UPDATE folder to your psp (if you already have an UPDATE folder, just move the eboot) and then run the update.
this will get the job done.
p.s. - dont worry about the ofw 5.00 eboot it mentions in the article. since you already have 5.00 m33-4 installed, you dont need it.