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!