In general, you can see the change background option when you start Google meet.
But if you don't see this option on Google Meet then you will have to check some options in Google Chrome and do some changes in Chrome.
Check your Chrome version - it should be 84 or higher on Mac, Windows, or Linux.
Chrome should also be the version of 64 bit. You can check it by clicking on the three-dot icon
then click on Help ----> About Google Chrome
Now after this, you should check that Use hardware acceleration should be turn on. To check this click on three dots in the right corner of your Chrome, then click on Settings then Advanced, after that System. Here you will find the option Use hardware acceleration when available, make sure that this option should be turned on.
Now after checking all this general information and settings and all are correct according to given, then you will have to check some other requirements.
Check that your browser supports WebGL.
To check open https://webglreport.com/, and check that Major Performance Caveat: should be "No"
If Major Performance Caveat: is "Yes" then change background will not appear on Meet.