Google: How to Mark All Unread Emails as Read in Gmail

To mark all unread emails as read in Gmail, please follow the steps below:
1. When logged-in to your Gmail account, in the search box up top, type the following, then press the search button: is:unread

2. Click the down-arrow in the "select" button, then choose "all."

3. The emails on the page you're on should now be checked; however, to select all of your unread emails, you need to click where it says "Select all conversations that match this search." It will be centered, just above the top-most email, as pictured below:

4. Finally, click the "More" button, then click "Mark as read."