1. Go to Company > Companies > Search for the company account code.
  2. Open the company record the select the Marketing tab.
  3. Here you will find 6 classifications, select the classification you wish to add to and in the drop down there is a + option to add.
  4. Type in the classification label and select Save & Close.

Classifications are multi-select, so each customer can have multiple labels against the Classification set. 

There are 6 dropdown lists in the Marketing tab which you can manage in this way. 

You can rename the Classification Names in the Administration module, in the User Defined Labels section.