1)Check what background apps use the battery the most in settings and put them to sleep and sometimes specific apps do that and if you don't need them that much then uninstall or delete them. 2)Change the battery. Im assuming it's a phone we are talking about and if you got a phone with a removable back then you buy the battery and change it yourself but if it's an iphone or a Samsung phone later than the s6 then you have to take it to a phone shop and let them change the battery because it usually includes removing the screen since they access the battery from the front.