What machine is it?
A lot of older machines won't support AIX later than 5.1.
The backup solution will depend on what backup devices you have, tape, writeable CD or DVD, a NFS drive, etc.
Local backups are best (unless you have a NIM server) because if the box won't boot you can still boot from the tape / CD / DVD and restore it - You can never be sure your backup solution works until you restore it!
If you can, take 2 mksysbs and a savevg for each vg, just to be safe.
As you are only doing a release not a version upgrade there is little to worry about.
If you have a spare disk or a mirrored rootvg you could investigate alt_disk_install.
http://publibn.boulder.ibm.com/doc_l...I0723971004MKM
Which will allow you to keep your current system and try the upgrade, if the upgrade fails just reboot the original system - still take the backup before you start though...