If an Office 365 group has the wrong SMTP address it's not something you can change from the WebUI but it can be done through powershell.
Make sure you have the latest Exchange module installed:
Install-Module ExchangeOnlineManagement
Then use
Connect-ExchangeOnline
complete the login as usual (logging in to the 365 tenant admin documented in 1 pass.
Add the new SMTP address:
Set-UnifiedGroup -Identity "Group Name" -EmailAddresses: @{Add ="GroupName@domain.com"}
Set the new SMTP address as the primary
Set-UnifiedGroup -Identity "Group Name" -PrimarySmtpAddress "GroupName@domain.com"
Remove the old (incorrect) SMTP address
Set-UnifiedGroup -Identity "Group Name" -EmailAddresses: @{Remove="OldGroupName@domain.onmicrosoft.com"}