The ornamental design for a boot upper, as shown and described.
FIG. 1 is a side elevational view of the boot upper; and,
FIG. 2 is a perspective view of the boot upper.
The portions shown in dash-dash broken lines are solely for the purpose of illustrating context, environment or other unclaimed areas and form no part of the claimed design. The dash-dot broken lines depict boundaries between claimed and unclaimed portions.