Exceljet

Quick, clean, and to the point

Apply or remove underscoring

Windows shortcut 
CtrlU
Mac shortcut 
U

This shortcut toggles underlining on and off for the current selection.

On Windows, Ctrl 4 also toggles underlining.

This is the best Excel training site I have ever seen. -Kedy
Excel foundational video course
Excel Pivot Table video training course
Excel formulas and functions video training course
Excel Shortcuts Video Course
Excel Charts video training course