Gmail – Mark All Emails as Read Before a Certain Date

There are two ways:

1. Perform a search to find all the messages you want to select. (You can use is:unread too)
2. Click select all
3. You will see that above the first item in the list there is a banner that says something like: All 50 conversations on this page are selected. Select all # conversations in "..."; click that link.

You can also do this with a filter, by creating the filter, choosing to mark the messages as read, and then choosing to apply the filter to the matching conversations. Then simply delete the filter.

select the label, in the search bar, it will show label:mylabel - add label:unread and search.

select all messages by ticking the top left checkbox

if there are more unread messages than can be displayed, a small notice will appear above the first message in the search results - you want to select all messages which match the search.

now you can click the 'more' button and select the 'mark as read' option.