Your question is confusing!!?? You can create a datum axis thru a datum as one of constrains, but if you want to align an existing one you have to break one or more existing constrains and add new ones.
You’ll find on the right side menu a square box with two symbols on it (perpendicular, and parallel symbols). Click on that one and the constrains menu pops up.
If you can’t see it, add it to your menu using “Utilities --> Customize screen --> Sketcher tools”
If you select your datum as a sketch reference and sketch the centerline right on the datum when your mouse snaps to it the constraint should be created automatically.