Quit Roblox if it is open
Open the App Store
Click Updates
Find Roblox and click Update
If Roblox was installed from the Roblox website, open Safari or your browser
Go to roblox.com
Sign in to your account
Join any game
Allow Roblox to download and install the latest version
If the update does not start, delete Roblox from Applications
Reinstall Roblox from roblox.com
Restart your Mac if needed