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Kingo 1 click root app
Also installs a rootkit that continuously communicates with servers in China, sending details on everything that is on your phone and everything that you do with your phone. Also disables / modifies any other security apps on your phone.
Most oneclick solutions are disguised spyware.
Unlock the bootloader, flash TWRP, flash SuperSU. Choose a SuperSU version that is a Chainfire version if you can, as the new versions are less trusted.
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I'd say the best way is to follow the xda thread for your phone. 30min or so with a pc and a usb cable and it will be done.
https://forum.xda-developers.com/nexus-6p/general/guides-how-to-guides-beginners-t3206928
If you do it, don't use an app would be my main advice. 90% of them will use that access to skim info or do other bad things for the phone. What phone is it? Default advice always used to be look on XDA, but its been a long time since I've bothered honestly