Buy a new CR2032 battery. To change the battery, find somewhere dry, away from dust and water. By hand or with a coin, turn the back cover anticlockwise. - Remove the old battery. - Put the new battery in the back of the watch, between the metal brackets with the negative side on the bottom, positive side facing up (marked with a + sign).- Replace the cover while taking care to not damage the seal. Lock the cover by turning it clockwise until fully closed. The cover should be completely flat and securely locked. If it is not, take the cover off and start again. You can also go to a Decathlon store or to a watchmaker.