I have recently replaced my iPhone 6 battery with an iFixit battery and everything works normally after iOS11.3 upgrade. The only problem I see is that the new Battery Health section does not seem to work with this battery, it does not show either Max Capacity or Max Performance Capacity.

The comments were very helpful in understanding how to remove the bracket from the mute button and then re-fix it using the soldering iron to melt the three plastic posts. However it was not clear for me from the comments how to remove the small metal plate that is that glued to the volume switches. And also what is the right procedure to refix this plate to the new cable. Since my problem is only that the up volume button is not working I decided to reassemble the phone waiting this guy to be a little bit more clear about this two topics.