To make changes in the description field for each line item in a Project in the Project click on Descriptions tab. You'll see the option to modify the Work Order, Quote or Invoice there. Changes here will only affect that invoice/Work Order/Quote.
Permanent changes can be made in Preferences > Forms
To make company wide preference changes: In Preferences module-> Forms you can set up what information you want to appear on your reports as a default.
You may also create Invoices unique to each customer in the customer module. This will over-ride your company settings in Preferences when this customer is selected in a Project.
In Customers -> Invoice Description Line Format