The ornamental design for a portion of a boot upper, as shown and described.
FIG. 1 is a right side elevational view of a portion of a boot upper according to the present invention.
FIG. 2 is a left side elevational view of the portion of a boot upper.
FIG. 3 is a rear elevational view of the portion of a boot upper.
FIG. 4 is a front elevational view of the portion of a boot upper; and,
FIG. 5 is a top plan view of the portion of a boot upper.
The broken lines on the interior of the portion of a boot upper in FIGS. 1 and 5 depict boundary lines while the remaining broken lines depict parts of the portion of a boot upper that form no part of the claimed design.