According to the release notes, this update should bring some much-needed improvements and bug fixes.

Just like iOS 5 beta 4, you can download it over-the-air. However, to do OTA, you must do this first on your device:
Settings > General > Reset > Erase All Contents And Settings
This will erase all the data on your device - make sure to backup first before doing this (if applicable). From here, you can use the OTA feature on iOS 5 beta 3+ firmwares.

You can also go the "original" route and just install iOS 5 beta 5 with the IPSW file. Make sure to do it in this order:
Backup/sync your device on iTunes 10.5 beta 4
Restore your device to iOS 5 beta 5
Update iTunes 10.5 to beta 5 and sync your backup with iTunes, or even iCloud

Supported Devices:

Remember, as discovered on iOS 5 beta 4, the OTA update detects components on your device if jailbroken and will refuse to update. In order for you to upgrade from a jailbroken device, you must restore to your current stock iOS 5 firmware, and upgrade via the IPSW or OTA.

Source- modmyi