The ornamental design for a sandal upper, as shown and described.
FIG. 1 is a perspective view of a sandal upper showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof; and,
FIG. 6 is a top view thereof.
The broken lines showing the sole of the sandal are for environmental purposes only and form no part of the claimed design, while the remaining broken lines depict stitching and are included in the claimed design.