If you actually mean "read" them the obvious answer is to open them one at a time and read them.
If what you actually mean is how do you mark them as read you can click the small box at the top of the list of emails and this will select all the visible emails and you then go to the menu at the top and select "more" then " mark as read".
There is a limit on how many emails can be selected so be prepared to being this for a long time.
An other option would be to install an email client/app and once all the emails are available on the email client you can select the Inbox and "mark as read" which will mark all the contents of the Inbox as read.
This option may take a while for all the emails to show in the email client/app but once there they can be marked as read all at once.
