Open Airport Utility. Option-Click on the Version Number in an Airport unit - you'll see the immediately-available firmwares you can install, plus an "Other" option that will offer a modal dialog for other, downloaded FMs.
Thanks for posting that. Not a single Mac in my house was performing automatic backups since updating to 7.7.7... I would have to restart the target Time Capsule then go to the Mac and choose "Backup Now" which would work, but all other machines would fail after that. Got tired of that after almost two months and just did a downgrade.