The Wallet.fromMnemonic function has a second argument to specify the BIP-32 derivation path. By default it will use m/44'/60'/0'/0/0, but if you want to get the second account, you can use m/44'/60'/0'/0/1 for example:
const { Wallet } = require('ethers');
const wallet = Wallet.fromMnemonic('one two three four ...', `m/44'/60'/0'/0/1`);

Alternatively, you can use the HDNode class to quickly derive child keys from a mnemonic phrase. For example:

const { utils } = require('ethers');
const hdNode = utils.HDNode.fromMnemonic('one two three four ...');

const secondAccount = hdNode.derivePath(`m/44'/60'/0'/0/1`); // This returns a new HDNode
const thirdAccount = hdNode.derivePath(`m/44'/60'/0'/0/2`);

The HDNode can also be used to get an instance of Wallet. This is also what Wallet.fromMnemonic does.

const wallet = new Wallet(hdNode);

https://ethereum.stackexchange.com/questions/84615/recover-all-the-account-under-mnemonic-using-ethers-js