Typing Overscripts
Typing 
Type the expression that will have an overscript.
Create an overscript by pressing the
Control key and the
7 key at the same time.
Type an underscore into the placeholder (which is already selected).
Move the insertion point out of the overscript position down to the baseline by pressing the
Control key and the
Space bar at the same time.
Typing 
Press the
Control key and the
7 key at the same time.
This creates a pair of placeholders, one on the baseline and another in the overscript position.
Type an underscore into the placeholder in the overscript (which is already selected).
Press the
Tab key to select the placeholder that is on the baseline.
Type the expression under the overscript.
Press the
Control key and the
Space bar to get out of the overscripted expression.