FIG. 1 is a perspective view of the ornamental design for a boot upper described and shown herein.

FIG. 2 is a left side elevational view thereof.

FIG. 3 is a right side elevational view thereof.

FIG. 4 is a front elevational view thereof.

FIG. 5 is a rear elevational view thereof; and,

FIG. 6. is a top view thereof.

The surfaces of the articles not shown in the drawing, and the portions of articles shown in phantom lines, form no part of the claimed design.