The Spec tab is one of the product tabs which appears with product details in each of these screens: the Catalogues screen, the Search screen or when you view Suppliers > Individual Prices
Open the 
Catalogues window and locate a product using the Search options to access these options on the Spec tab
If the product has a specification within the catalogue you can amend it, or add your own to a blank specification.
Click the 
Edit button and edit the text in the text box.
Click 
Update to save
Clicking Reset returns to the catalogue specification, or a blank box if there wasn't one.