In SketchUp 101, I suggested that every item (board, plank, box, etc) you create be turned into a Group so that you didn’t mistakenly edit it while trying to edit something new. The Group basically locks the object’s size and color while leaving you free to move or copy it. But what happens when you DO want to edit the Group?
Select the object, then right-click and click on Edit Group in the context-menu:
Now the rest of the scene is greyed out, and you are allowed to edit ONLY the selected object:
Simply make whatever edits you need, then use the main program menu to close the group. This will return you to the full scene, with your changes made to the group.