The HTC One M8 battery is irremovable and bury underneath the motherboard, so if you want to replace a new one, you have to tear down the phone. You need to separate the M8 screen from front housing, then disassemble the battery. Here is step-by-step tutorial on how to change a HTC One M8 battery for your reference. http://www.etradesupply.com/blog/fix-a-cracked-htc-one-m8-screen/
Here is a very good tutorial walking you through how to step-by-step replace a cracked Galaxy S5 display http://www.etradesupply.com/blog/how-to-replace-cracked-samsung-galaxy-s5-screen/ Please kindly note: Samsung Galaxy S5 comes with the new water resistance performance. However, if your Galaxy S5 suffers breakage and need to be repaired, you have to be aware of some non-reusable parts that must be replaced to the new one when reassemble. These parts are: Galaxy S5 front housing adhesive / form tap Galaxy S5 Bracket Cu tape 10 screws on the middle frame 3 screws on the rear housing.
You can update the iOS to the latest version to fix the disabled iPhone 5 Wi-Fi switch issue. But if you don’t want to upgrade the iOS, you can try the following solution first. Go to Settings > Disable Airplane Mode > Go to General > Rest > Tap “Rest Network Settings” (Please kindly note all saved Wifi passwords, Bluetooth pairing records, VPN as well as the APN settings will be removed after this setting) > Press and hold the Sleep and Home Buttons together to restart the iPhone If this does not work, you also can try other solutions mentioned at http://www.etradesupply.com/blog/solve-iphone-5s-iphone5-wifi-issue/
If your galaxy s4 screen was cracked and the power button is broken too, you can purchase the replacement parts and replace it yourself by following this tutorial http://www.etradesupply.com/blog/how-to-repair-cracked-galaxy-s4-screen-assembly/ It is quite easy and can save your money!
It depends on what kind of cellphone you hold. Some phones' touch screen can be separated with the LCD screen, so you can fix only the touch screen yourself. But some phone's touch screen, such as Samsung Galaxy series, is hermetically attached the LCD screen, so you need to replace the whole LCD screen and touch screen digitizer assembly together.