In WordPress 2.3.X :
Go to Manage > Category Icons and click on Icons tab.
In WordPress 2.5 :
Go to Settings > Category Icons and click on the Icons tab.
In WordPress 2.7+ :
Go to Posts > Category Icons and click on the Icons tab.
If you want to delete the icons of a category and its priority, click on the checkbox in front of it, and then click on the button “Delete Icons and Priority”.
In the “Name” column, click on the name of the category to edit or select its icons.
In the dropdown menu icon or Small icon, select an icon among the ones you uploaded in the default folder (wp-content/uploads) or the folder you’ve choosen to upload them. And then click on the button to the right to validate.That’s all.