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 elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines showing the sole of the sandal depict environmental only and form no part of the claimed design while the remaining broken lines depict stitching and are included in the claimed design.