By mistake, I began the update of my phone and now it's very slow; I want to stop the update.
I rebooted the phone but it still shows the update process.
Please, suggest me some idea to stop the update process.
I'm assuming you are using Windows 10 mobile update right now, so for you to go back, you should use the Nokia Recovery Tool.
You can download the tool in your PC from here.
Connect your phone and it will revert the update.
Be aware that all your phone data will be erased, so make a backup first.