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 top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is an enlarged front perspective view thereof;
FIG. 5 is an enlarged rear perspective view thereof;
FIG. 6 is a right side elevation view thereof; and,
FIG. 7 is a left side elevation view thereof.
The broken lines applied to the sandal upper strap represent claimed stitching. Broken lines in or showing the sole or toe post of the sandal are for environmental purposes only and form no part of the claimed design.