Many ways to return to inbox
To return to the inbox, you can of course click on the "Inbox" link on the left had side, or if you're in conversation view, clicking on the "Back to Inbox" link above the conversation. There is also green 'Inbox' label text next to the subject. But you can also click on the Gmail logo, a much larger target, to go back to the inbox.
Keyboard shortcut users can type "g i" to Go to the Inbox.
Clicking on the logo, the Inbox link, the Refresh link, or the keyboard shortcut "u" all will refresh the inbox with the latest messages received.