The ornamental design for a sandal upper, as shown and described.
FIG. 1 is a front right 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 right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is the top plan view thereof; and,
FIG. 7 is the bottom plan view thereof.
The broken lines in the drawings depict portions of the sandal upper that form no part of the claimed design. In particular, the Sandal Sole forms no part of the claimed design.