On the computer, when in a thread, out to the right you see the scoll bar that has the post #'s listed out. At the bottom of that scroll bar, there is a reply icon, and an icon that is a circle. If the circle is blue, that means you are tracking that thread. Click on it, and a new box will open allowing you to select normal or mute or whatever option you want to change it to.
On your phone, at the very bottom of the screen there is the post count down there that may say 17/200 or whatever, showing which post you’re on. Tap on that box and a new screen will come up allowing you to either select which post you’d like to jump to, and also giving you the option to change your tracking of the thread.