When you open an email, it automatically marks as ‘read’. But you can also mark-as-read a batch of emails in all (or chosen) folders - just in a few clicks!
Search mailsearch box and press Enter.
Select all conversations that match this searchbutton. Click it.
Morebutton (looks like three vertical points/dots).
Mark as readoption will appear. Click it.
Here we go! All messages are marked as read now.
If you want to mark as read only the emails from the Inbox folder (and skip other folders),
label:inbox is:unread instead of
is:unread in the search box
(as described in step 1 of this manual).
Using this approach, you can not only mark emails as read, but also perform any actions shown in the last screenshot (Mark as unread, as important, as not important, add star, remove star, etc.).