In the main navigation, you can find the inventory adjustment by going to Customers > Other Transactions > Inventory Adjustment > New.

Then fill in the top section's required fields. In the customer field, search for your name.  In the Adjustment Account Field, type in the course number; when the course is found in the dropdown, select it. The Custom Form field should already be filled in, and the date field as well. 

Next, in the Adjustments section, type in the course number and add a W in front of it. You should then see the inventory number that is associated with the course appear in the drop down. Select the inventory that you want to adjust. The quantity on hand column will show what is currently still available to be purchased. In the ADJUST QTY. BY column, you will either be adding or removing seats. To remove a seat, just place a minus before the number that you want like so: -1. To add a seat(s) just type the number like so: 5. To do multiple inventory adjustments at once, click the add button and repeat the same process as before.

NOTE: You will need to factor in all of the seats that have already been purchased into calculations. This way, you won't run the risk of over selling seats for your course. Unfortunately Netsuite does not add that as column on this page.