You need to do this since we're on 1.16.5 and Minecraft latest version is 1.17
In the tab list at the top of the launcher, there's a tab called Installations. Click that and select + New
Here you can name your new installation and select the version you want to downgrade to in the dropdown. Once down, click Create in the bottom left-hand corner
Now that you have done that you can return to the Play tab, select your new installation in the dropdown and you're all set.