How do I archive a Conversation?
Archive old messages to hide them from the Inbox and clean things up a bit.
Select the Conversation
Select the conversation.
Select Multiple Messages
To select multiple messages to archive, click the command key (Mac) or the control key (Windows) while clicking each message you want to select. When multiple messages are selected, the right panel displays a multiple conversations message icon. You can also use the same key command to deselect a message.
To select all messages, click the command + A keys (Mac) or the control + A keys (Windows).
To select a range of messages, click the first message you want to select, hold down the Shift key, and then click the last message you want to select. All messages between the first and the last messages will be selected.
View Archived Conversations
Inside of the Archived messages, click the message  you want to unarchive. Click the Unarchive button  to unarchive the message and move it to your Inbox.