We need some more information!
There are two completely different ways of sending and receiving email. In most offices, for example, email is handled through a dedicated program (called an email client) such as Outlook, Outlook Express, Windows Live Mail or Thunderbird. Many home users also use email clients but a growing number prefer to access their email accounts through their web browser, such as Internet Explorer, Chrome, Firefox or Safari. Without knowing which method you're using to access your mail, it's impossible to provide an accurate answer to your question.
However I'll take a guess that you're using Outlook Express or a similar program (simply because I know that Outlook Express has a nasty habit of mixing up usernames and passwords). If so, go to Tools > Accounts. If your email account isn't already highlighted in the list, click on it. Then click on Properties. Click the Servers tab,delete the account name and password, then re-enter them. Click on Apply and then on OK.