Either you can manually add it using the following details:
- Network name: Arbitrum One
- New RPC URL: https://arb1.arbitrum.io/rpc
- Chain ID: 42161
- Currency symbol: ETH
- Block Explorer URL: https://arbiscan.io/
Or just simply:
- Go to ChainList and connect your MetaMask Wallet.
- Look for 'Arbitrum One' in the search bar at the top of the page.
- Tap 'Add to MetaMask' and the verified RPC information will be automatically added to your extension.