Get a new back cover for the SL500 watch available on the Decathlon website. Remove it while taking care to not damage the seal. - Turn the watch’s back cover anticlockwise by a quarter turn by hand or with a coin. Remove the cover.- Reposition the battery if necessary.- Place the new cover by aligning its dot with the watch’s open lock.- Turn the cover by a quarter turn clockwise until the dot lines up with the closed lock.- Make sure the cover is secure and perfectly flat. You can also go to a Decathlon store or to a watchmaker.