The ornamental design for fold-up backpack, as shown and described.
FIG. 1 is a perspective view of a fold-up backpack in a use configuration;
FIG. 2 is a front elevation view of the fold-up backpack in the use configuration;
FIG. 3 is a rear elevation view of the fold-up backpack in the use configuration;
FIG. 4 is a left side elevation view of the fold-up backpack in the use configuration;
FIG. 5 is a right side elevation view of the fold-up backpack in the use configuration;
FIG. 6 is a top plan view of the fold-up backpack in the use configuration;
FIG. 7 is a bottom view of the fold-up backpack in the use configuration;
FIG. 8 is a perspective view of the fold-up backpack in a storage configuration;
FIG. 9 is a front elevation view of the fold-up backpack in the storage configuration;
FIG. 10 is a rear elevation view of the fold-up backpack in the storage configuration;
FIG. 11 is a left side elevation view of the fold-up backpack in the storage configuration;
FIG. 12 is a right side elevation view of the fold-up backpack in the storage configuration;
FIG. 13 is a top plan view of the fold-up backpack in the storage configuration; and,
FIG. 14 is a bottom view of the fold-up backpack in the storage configuration.
The broken lines on claimed surfaces depict stitching and form no part of the claim. All other broken lines depict portions of the article and form no part of the claim.