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 thereof; and,
FIG. 3 is a rear elevational view thereof.
The dashed broken lines indicate unclaimed environmental structure. The dash-dot broken lines adjacent to the full lines represent boundaries and are not part of the claimed design.