When replacing a battery make sure to rid the old battery responsibly.You will need the plastic separator tool or if you don't you can use your nails but it is highly not recommended as you could get injured.
First you need to remove the back case of the phone.
Place the phone face down.
Using the plastic opening tool, insert into the initial gap shown.
Slowly guide the tool around the case until the case comes off with ease.
Place the back case to the side.
Use your finger or the opening tool to remove battery.
Remove the battery carefully to not damage the phone or the battery.