Warning! This is a tricky procedure and not recommended for everyone. If you are not sure what you are doing or value your iPhone 6 immensely, it would be better to take your phone to the nearest Apple Store for diagnosis.
Apple iPhone 6 Disassembly Instructions:
1) Turn off the phone. Remove the SIM Card using the tool.
2) Remove the pentalobe screws (one on either side of the charging port).
3) This loosens the front display assembly. Use a case tool to gain some leverage. Use gentle force to avoid cracking the display. Do not tear it, because the display is connected to the board via cable.
4) In the top corner, there is a metal plate covering the cable connection for the display. There are five Philips screws on this plate, which can be removed. After this, the ribbon can be disconnected.
5) Many of the components are located on the front LCD panel of the phone which have been just removed, making it easier to replace them. The earpiece is protected by metal plate and can be accessed in a similar way to the ribbon cable.
6) This front assembly also protects the front facing camera, which can be removed.
7) If you have a defective home button, this is located at the bottom of the phone and protected by another silver plate. There is a very small ribbon cable to left of the home button. It is easier to remove this if you apply a little heat which will melt the glue slightly. After this the button can be removed.
8) The LCD is held in by the large metal plate, this in turn is fixed in by seven Philips screws around the frame edge.
9) There is one more ribbon cable connector, which lies next to the home button. Be sure to undo this before taking the screen out.
10) If you need to replace the battery, this is accessed on the main board of the phone. Lying next to it, the battery ribbon cable is protected by another metal plate which has two Philips screws which should be released. Once done, you can remove the cable, and take out the battery, fresh for replacement. In a change from previous models, the battery is glued into the phone so you may need a heat source to get some leverage going.
11) This covers most of the main components on the iPhone 6. Most of the other components sit neatly on the rear frame and all can be accessed. For further disassembly, watch this video here: