The ornamental design for a boot upper, as shown and described.
FIG. 1 is a left side elevational view of a boot upper according to the present invention;
FIG. 2 is a right side elevational view of the boot upper; and,
FIG. 3 is a rear elevational view of the boot upper.
The broken lines depict optional features that do not form a part of the claimed design. The inventive design encompasses the embodiment of a boot upper illustrated in the drawings, as well as an embodiment which is the mirror image of that shown.