In Deleting or Editing an Asset, you have to Rollback Depreciation from the First Year/Date.
Go to "Tools" and click "Depreciation page"
And select "Rollback Depreciation".
Choose the date of the start of the period prior to the period the asset was added (i.e. if you added the asset on 1 Feb 2018, then you'll need to select 1 July 2017 i.e. start of financial year) , then click "Rollback"
You have to delete "Asset" before deleting "Asset Group".
Click the three little dots and choose either "Remove" or "Edit".
Now, you can remove or edit the "Group Asset".