The ornamental design for a portion of a boot upper, as shown and described.
FIG. 1 is a left side elevational view of a portion of a boot upper according to the present invention.
FIG. 2 is a right side elevational view of the portion of a boot upper.
FIG. 3 is a front elevational view of the portion of a boot upper.
FIG. 4 is a rear elevational view of the portion of a boot upper; and,
FIG. 5 is a perspective view of the portion of a boot upper.
The dash-dot broken lines represent boundaries between areas that are and are not part of the claimed design. The dashed broken lines depict parts of the portion of a boot upper that form no part of the claimed design. The claimed design is for a portion of a boot upper as illustrated in the drawings, as well as the opposite matching portion of a boot upper, i.e., the mirror image of that shown.