For the great majority of times you open up SAP for editing transaction SCC4 will do the trick.
However if you ever find that someone in your team needs SAP opened up for direct editing and they keep getting a message from the system saying "SAP System has status 'Not Modifiable' " This means you need to open up the system even further.
While this is dangerous because it opens up all sorts of potential issues you can do it in these kinds of circumstances using transaction SE03.
Go into SE03
Go down to the Administration part of the left hand menu system
Double Click on "Set System Change Option"
Change Global setting from "Not Modifiable" to "Modifiable"
Save settings.
Get the person to do their work then change it back immediately. - If auditors find this open in your system expect a chewing out so make sure you follow up on this and check it.
