The ornamental design for a boot upper, as shown and described.
FIG. 1 is a left side elevational view of a boot upper, with the boot's shaft fully extended;
FIG. 2 is a right side elevational view of the boot upper, with the boot's shaft fully extended;
FIG. 3 is a rear elevational view of the boot upper, with the boot's shaft fully extended;
FIG. 4 is a left side elevational view of the boot upper, with the boot's shaft folded down;
FIG. 5 is a right side elevational view of the boot upper, with the boot's shaft folded down; and,
FIG. 6 is a rear elevational view of the boot upper, with the boot's shaft folded down.
The portions shown in broken lines are solely for the purpose of illustrating context, environment or other unclaimed areas and form no part of the claimed design. The inventive design is intended to encompass a boot upper as illustrated in the set of drawings, as well as the mirror image of that shown.