Unfortunately, this can happen from many things. The device being charged to often, the device being left in the heat for too long, the device getting moisture in it, the batteries power has decreased with time, etc. I would try resetting the device to factory settings, you may have certain apps or settings applied that take more battery to function. If resting the device does not change the batteries tendencies in the next few days, it is probably time to change the devices battery.
The screen, and screen bezel not only have a slight adhesive keeping them in place, but also a compression fit. With your plastic trim tool ( flat plastic edge for getting into grooves and applying pressure without risk of damaging device ) slide the flat edge under the screen bezel, this will be easier towards the smaller width sides of the screen. Gently apply pressure up, to remove the screen and black bezel from device housing. This pressure will not be gentle so do not be afraid of the device breaking, when popping the screen and bezel out.
Yes, unfortunately the battery in this device is soldered in place. To replace the battery gently tug on the old batteries soldered connection to break the wires free, making sure not to pull the wires out of the motherboard. Once here make sure the tips of your wires, no more than half a centimeter, are clean without fray or corrosion. Now, you must solder the new battery in place with a soldering gun. Route the wires in the same fashion as the original battery. Once all connections are complete, sealed, and cooled down, it is time to but the device back together. Now try restarting your device, and enjoy the extended battery life!