Assuming that you are using webmail via a web browser. Log onto your email account and at the top of the list of emails you will see a small box to the left of the "From" header. Tick this box. That should select all your emails and put a tick next to them in the list. Now go to "Actions" in the menu bar above the list. Click on that and you will get a drop down box. Select the option " mark all as read".
This should mark all your email as read and should stop it showing as having 1 unread message.
If you are using an email client/app there will be a similar setting but without knowing which email client/app you are using it is not possible to give more specific details.