On the “Chat” page, select a group or a private chat and swipe left on the name. The options to “Mute” or “Pin” or “Delete” will appear. Simply click on the action you want to do.
You can also enter a group and tap on the “…” in the upper right corner. There you will see the “Mute Notification” option as well.
If you want to unmute or unpin a group or a private chat, swipe left on the name again and you can click on “Unmute” or “Unpin.” Once you click on “Delete,” the group or the private chat will be deleted from your “Chat” page.