Depending on what you're after, one of these alternatives will suit you best;
One by one
From the main table, click on the message and you’ll be able to read the complete message.
If you’d like to mark as read more than one message at the same time, from the main table, check the messages you’d like to mark. If you’d like to mark ALL messages, simply click on “Select All’ at the bottom of the table.
Once you've selected the messages you’d like to mark, click on "mark as unread" button at the bottom of the table.
All you have to do is go to Settings > Rules & Triggers and create a rule so that all/ some inbound SMS that match certain conditions are marked as read.
To find out how, you can check out this step by step article.