You can't currently change your Argent username.
You would need to create a new wallet with a new username, and this would mean you lose access to your wallet. This also means that you will need to pay the wallet activation fee for the new wallet if you intend on using Argent Vault (Ethereum mainnet wallet).
If you'd like to go ahead with this, please move your assets out of your old wallet temporarily. Then contact firstname.lastname@example.org to help with the new wallet.