Unscrew the screws from the underside of the plastic handle. You can flip the cart upside down if it is easier, or simply reach underneath to unscrew them.
Check the threads on the screws to ensure that neither one is stripped. If a screw is stripped, replace it.
Remove the plastic cover from the cart handle.
Apply a small amount of hobby glue to the metal rail along the top of the handle.
Press the top plastic piece back into place over the metal handle. The glue will help hold the plastic piece in place more securely.
Hold the bottom plastic piece in place along the bottom of the cart handle while you insert each screw and secure them in place with the screwdriver. Tighten them fully so they don't work themselves loose with use.