You should never replace 15-amp fuses with 20-amp fuses. If you use the incorrect amp fuse, then you have a serious risk of an electrical fire.
The higher-amp fuse allows you to draw more amps from the fuse, but the wiring is only suited for the lower-level amp. Eventually, the wires heat up, which heats the insulation, and then a fire may start in the wall.
Wall fires also have the added risk of burning plastic materials that release toxins. These toxins could render people unconscious, so you would be unable to get away from the fire caused by using the wrong fuse.