I have an IBM P520 that I need to upgrade from 5.2 to 5.3. My background in Unix is from 15 years ago so I don't remember much. IBM documentation is hard to find. Any help would be appreciated.

The best way I have found to search the IBM web site is to google with:
site:ibm.com
at the beginning of the search line, so try:
site:ibm.com aix migration 5.2 5.3
for instance.

As with any AIX changes, take TWO mksysb's first, then you can't go too far wrong.

If you have more than 50 SAN disks, disconnect the SAN / fiber before you try the migration.

If you have SAN, check the procedure to migrate AIX in the associated SAN driver doc's, for instance with SDD / SDDpcm you must vary off the SAN volume groups, remove the SAN devices (rmdev) and uninstall the SAN drivers, then migrate AIX, then reinstall the newer drivers and reconfigure the devices. The drivers and host attachment scripts are AIX version specific.

If you try booting the 5.3 disk and don't get the migration option check here:
IBM - No Migration Option

On the 5.3 CD there is a premigration script, run this to check out your system first.

Make sure the system firmware is up to date first:
http://www14.software.ibm.com/webapp/set2/firmware/gjsn

Probably a good time to get the device microcode up to date too, see above link.

You may be able to just boot the 5.3 disk and migrate but a few checks first could save you a whole world of trouble.

At the very least take a mksysb before you do anything.