I assume there is no film cassette in the camera. Take a heavy weight paper clip and straighten the open end outward. Using a pair of pliers bend 1/8 inch of the end of the wire 90°, it should look like a L. Insert the L through the opening then turn the short end against the outside edge of the camera and pull up. The back should open.
Put any loose washers back on the rewind shaft then put the shaft through the hole in the camera top. Thread the rewind knob onto the shaft then hold the shaft stationary and tight the knob firmly.