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 an enlarged front elevation view thereof;
FIG. 5 is an enlarged rear elevation view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
Broken lines in or showing the sole of the sandal in FIGS. 1-7 and broken lines in or showing the middle\side and rear portions of the ankle strap of the sandal upper in FIGS. 1-6 are for environmental purposes only and form no part of the claimed design. The Claimed Design is the front of the sandal upper including the front portion of the ankle strap and the toe post.