The parameter is part of the Delve profile page, it's not exposed directly in Azure AD. The syntax you used above is correct, but you are likely missing permissions. To update this property on another user, you need to have Sites.ReadWrite.All (delegate) permissions added (application permissions are not supported). So try this:
Connect-MgGraph -Scopes User.ReadWrite.All,Sites.ReadWrite.All
Update-MgUser -UserId f20f5eb1-aabb-431d-a03b-3fbe5281f045 -AboutMe "aaa bbb ccc"
As to where to find the property, here's a screenshot from the Delve page: