Are you looking for information on how to update an incident using get-scsmobject, or what is the goal for using the get-scsmobject cmdlet?
To get the class, you'll want to run the following:
get-scsmclass -name system.workitem.incident | where {$_.displayname -eq "incident"}
Using this in the get-scsmobjet cmdlet:
get-scsmobject -class (get-scsmclass -name system.workitem.incident | where {$_.displayname -eq "incident"})
This will retrieve all incidents in the system. If you want to filter on a specific IR, you'll want to add the filter switch:
get-scsmobject -class (get-scsmclass -name system.workitem.incident | where {$_.displayname -eq "incident"}) -filter "Id -eq IR9999"
Hope this helps!