Forgive this question, but my searches haven't returned much specific to this situation.
Simply,
Can you upgrade a RB450 routeros from v 5.12 directly to version 5.21? Is it more adviseable to upgrade in steps or can it be done directly with no major problems? If it's advised to do so in steps, is there a location I can download the older mipsbe packages (ex: v 5.13, 5.15, etc)? The routerboard firmware is currently at 2.39 and does not show a newer version available.
Thanks for any replies.

You don't need to do it in steps. Just upload this file, and reboot: http://download2.mikrotik.com/routeros/ ... e-5.21.npk

Excellent. That is what I was hoping for. I already have the npk. Thanks Normis.