I haven't done it but
@fenderman says
here that this below will work:
- Go download and run the full installer of V5 from Blue Iris ==>> here.
- Then run the desired update (5.3.3.8) and it will revert back
Do you have your BI license info handy?
Any time you decide to uninstall/re-install BI it's good to go BEFORE YOU UNINSTALL to BI "Settings" => "About" and click on "Email support" button at bottom-right to copy to clipboard, then paste text into Notepad, Wordpad, etc. and save the file as text; then you can copy/paste into the new install of BI when it asks for your license key.