The ornamental design for a boot upper, as shown and described.
FIG. 1 is a perspective view of a boot upper according to the present invention.
FIG. 2 is a right side elevational view of the boot upper.
FIG. 3 is a left side elevational view of the boot upper.
FIG. 4 is a front elevational view of the boot upper; and,
FIG. 5 is a rear elevational view of the boot upper.
The broken lines are solely for the purpose of illustrating unclaimed areas and form no part of the claimed design. The claimed design also is intended to encompass the opposite matching boot upper, i.e., the mirror image of that shown.