If you need to delete it to Increase the amount: 

Manually add an additional Charge on the Student's record to account for the increased price of the class.
OR
1. Open the Course this line item originated from. This is found in Records, Courses
2. Select the Billing tab for the course and increase the price here.
3. If no price is given on the course, go to Configuration, Products and billing items 
4. Open the Billing item used on the course and update it's amount.
5. To update the student's Invoice, go to Administration, Generate Transactions Automatically
6. Open the parameter that was used to create the invoices initially.
7. Mark the box to recalculate at the very top right.
8. Preview/Generate the transactions and the invoice will be updated accordingly based on the new pricing 
 

If the student is dropping the class:

You may want to get rid of that line item. Dropping them within the dates provided on the Reversal Schedule will automatically create an offsetting credit or just remove/update that line item if it's unposted.  If you've already dropped the student and this didn't happen, it's likely that the date selected wasn't in the Reversal Schedule or you don't have a Reversal Schedule set up to automatically handle billing for drops.  

For more information on this topic see: How to set up Reversal Schedules

Outside of dropping the student within the acceptable timeframe, you have a few other options.

 

I. Add a Credit to the student for the amount of that line item and apply it to that particular line item.

OR

II. "Return" the Invoice Line item
1. From within the Invoice, Select the Transaction menu (at the top)
2. Choose to reverse a portion of the invoice.

You will see a message that says that the invoice will no longer be updated as a result of running the transaction generation process.  The means that if you change the amount that is billed for a course, the billing item on a course, or the determination table and generate again, this invoice will not be updated to reflect those changes.  That is usually not an issue, but just be aware that you will have to manage this invoice manually going forward if you choose this route.

3. After clicking Yes, you will see a screen appear for the return. 
4. Find the line item you want to reverse, enter the quantity to reverse.
5. Delete the other rows you don't want to reverse.
6. Save this return.