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 front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines applied to the sandal upper, which may appear in any of FIGS. 1-2, and 4-7, represent claimed stitching. Broken lines in or showing the sole of the sandal, including surfaces of the insole and the outsole, in FIGS. 1-7, are for environmental purposes only and form no part of the claimed design. The claimed design does not include any part of the sole.