See also the official documentation:
Help Center > Members Manual > Controlling the volume of email messages you receive from a group
The Groups.io home page provides a place to manage the topics you've muted or followed in all of your group subscriptions. The default home page show a list of all of your group subscriptions.
To get there click on the Groups.io logo in the upper left corner.
In the left column you can select a page showing all of the topics from all of your groups, or just those topics you've Muted, or just those topics you've Followed.
To change your options for a specific topic find it in one of those lists and click on it. Then you can use the buttons at the top of the topic display to change its selection options.