How do you disable your anti virus to update to 5.1?

The iPad doesn't have antivirus, so it won't be on there. If you are downloading the update via your computer's iTunes then how you disable you antivirus and firewall software will depend upon what the software is - Kaspersky, Norton, McAfee, ... ? If you know whose it is then somebody on here might have the same software and be able to tell you.

You disable antivirus and firewall on computer

For Kaspersky, I just right-click on the "K" on the bottom right of my computer and select pause protection and then select resume on my command.