Galaxy s6 won't stay charged.

Well seeing as the S6 is now a 5 year old device, it’s probably time for a new battery. You should be able to order a kit on ifixit and do the repair yourself or take it to a local independent shop. If you do the repair yourself, be sure to have an extra back glass on hand as it can get cracked during removal but it’s a relatively inexpensive part to replace. But do be very careful when separating the midframe. You can damage the display or peel the paint off at the top. If the paint comes off that’s just cosmetic and won’t hurt the device. There is also a cut out in the mid frame under the battery. You can damage the display through said cutout.