Lay the spirit level on the bottom shelf of the refrigerator, turning it at various angles to determine in which direction the refrigerator is sloping. Make a note of this.
Try levelling the refrigerator. Empty the contents and check for screws going down into the refrigerator legs through the refrigerator floor with arrows marked "Raise" or "Lower" or words to this effect which indicate that the level can be altered by turning these screws. These screws may be concealed by panels which can be levered open with a screw driver. Turn these screws, raising the legs of the refrigerator in the corners where the spirit level shows it is too low.
Make a quick fix by cutting small strips of foam and taping them to the door and door frame, both at the hinge and the closing edge. Make them thin enough that they do not stop the door from sealing properly when it closes. The strip at the closing edge will soften the impact while the strip at the hinge can jamb the door slightly to stop it closing without an extra push.
Try swapping the door hinge onto the opposite side. This may make the door swing open instead of shut but this is less of problem because you hold the refrigerator door when you open it. This way you can bring it to rest in the open position and it will stay there rather than swinging closed. Empty the contents of the door shelves, unscrew the bolts or screw which connect the hinges in the door to the body of the refrigerator and connect them again to the hinges on the other side of the door. This may be concealed under a panel, which can be lifted with a screw driver.
If the level cannot be adjusted from the inside, lay the refrigerator down on its side, and take note as you do this of where the legs stand on the floor. They usually will have left a mark in the floor but if not make one with a pencil. Check for a bolt or screw which allows the leg lengths to be adjusted from outside; if found, adjust accordingly. If not, place squares of wood where the appropriate leg or legs will stand to raise the low side of the refrigerator and make it level.
Stand the refrigerator back up and check with the spirit level to see if now stands level. If not bring it back down to its side, make adjustments and check again.