Before disassembling your phone, discharge the battery below 25%. A charged lithium-ion battery can catch fire and/or explode if accidentally punctured.
Insert a spudger or your fingernail under the battery at the small recess on the lower edge.
The top and bottom frame section hold very tight to the mid frame. It is possible to pull them off with your hands. If you're not able to do so, use the following removal procedure:
Use the flat end of a spudger and apply pressure to one of the corners of the frame section until you create a small gap.