Start by peeling off the copper tape heat sink (1) - it can be re-applied once the new battery is in place

Then remove the cover for the charging port and microphone (2) by removing the 3 marked screws

Unplug the battery (middle pink circle) using either your fingernail or a plastic spudger

remove the SIM tray by pressing a pin at the marked place

Then you can unscrew the two remaining screws (pink, 3) and remove the mainboard. Be careful to lift it from the bottom, since at the top the 3,5mm jack is fitted into the back cover. Keep those two screws separate, since they are a different length

Note: You can unclip the flex cable for the button, but the one for the display seems to be glued to the board