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 an enlarged front perspective view thereof;
FIG. 3 is an enlarged rear perspective view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right 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 in FIGS. 1-7 represent claimed stitching. Broken lines in or showing the sole of the sandal in FIGS. 1-7 are for environmental purposes only and form no part of the claimed design. The claimed design does not include the sole of the sandal.