If you click (or tap, if you’re on a device with a touch screen) on the Zoom window (where the video feed is), you should see several buttons show up at the top and bottom of the window.
Muting:
In the lower, far left of your window, you should see a little microphone icon. 
Clicking on that microphone will mute you. It should then have a red line through it.
Clicking on it again will unmute you.
Gallery/Speaker View:
If you touch/click on the window again to bring up the buttons, you’ll see a button in the upper right corner that says “Gallery View”.
Clicking on that will show you bigger pictures of the other people on the call. Then button will say “Speaker View”.
Clicking on it again will switch it back to a bigger, main picture of just whoever’s speaking.
Leaving the meeting:
In the lower right corner of your screen, you should see the word “Leave” in red letters (it’s a little harder to see on the dark background).
Clicking on that will allow you to leave the meeting.
To rejoin the meeting, you’ll have to click on the original Zoom meeting link again.
