To do so on a PC:
- Close the MTG Arena client
- Open a Windows command prompt (cmd.exe) as an administrator
- Type ipconfig /flushdns and press enter
- Type ipconfig /registerdns and press enter
- Type ipconfig /release and press enter
- Type ipconfig /renew and press enter
- Type netsh winsock reset and press enter
- Restart your computer and launch the MTG Arena client
To do so on a Mac:
- Open a terminal window from Applications -> Utilities. You could also press Command + Space and searching for "Terminal"
- From there, input the command for flushing your cache. The command you use is different depending on your Mac OS version.