The ornamental design for a sandal upper, as shown and described.
FIG. 1 is a left side elevational view of the sandal upper embodying the new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a rear elevational view thereof; and,
FIG. 5 is a front elevational view thereof.
The small dashed lines represent claimed stitching. The dash dot dot dash lines illustrate environmental structure and form no part of the claimed design.