As your update is freezing at 25%, I believe this article would be of great help:
Sling Media Technical Support
DON'T USE A MAC.
If you are trying to update your Slingbox and it keeps freezing at 25 percent, get a PC laptop and that should solve your problem.
I know, because I tried three modems, fought with my provider over the opening of Port 5001 and ultimately switched internet providers. Nothing worked until I borrowed a PC laptop, hooked it up to my home network, and within minutes everything was up and running again.
In the interim I called Slingbox and the guy assured me it makes no difference whether I'm on a Mac or PC. That's absolutely not true. I was ready to replace a perfectly good Slingbox for no reason.
Here's the other annoying part: After solving the problem I called Slingbox back to suggest they should not tell people the Mac situation is not a factor. At first the guy said they were aware of the issue and are trying to solve it. Except I called him out on it, pointing out the issue has been ongoing for more than two years. He stumbled through answers but still said they're trying to solve it. We ended the conversation before I could inform him how high on the priority list this problem must be for Slingbox for it to take this long to fix a bug.
Why they can't at least be up front with people (outside of wanting to sell more Slingboxes, of course), I'll never know.