My iPhone 6 plus was going slow, so I got a battery from ebay and followed the ifixit iPhone battery replacement tutorial – my phone is now much faster (it feels like a new phone again)

starting – all my tools ready

screen open

screen detatched

battery removed

new battery tape

new battery fitted (notice the protruding connector on the roght of the battery – I had to fix that

closeup of the fixed connector

screen attached

booting back up!