You can change it after creation, but you need admin permissions for that. If you do have them, go to the M365 admin center > Groups > select the group > hit Edit under Email addresses in the right pane. Or use the Set-UnifiedGroup cmdlet: https://learn.microsoft.com/en-us/powershell/module/exchange/set-unifiedgroup?view=exchange-ps
Also, you shouldnt be including the domain part (@keyman .com) in the name.