If you have previously installed the Desktop Radix Betanet Wallet, whenever you open the wallet it will try to load the saved wallet keystore.
If you need to start with a completely fresh wallet, or if you have forgotten your pin or password, you will need to delete the wallet.json file that is saved on your device.
You can find the wallet.json file from the following file path:
Mac: ~/Library/Application Support/radix-olympia-desktop-wallet
Linux: ~/.config/radix-olympia-desktop-wallet or $XDG_CONFIG_HOME/radix-olympia-desktop-wallet