You can change the title of the heading, but you have to realize it will change it globally. In other words, it's not from that point forward. If you go back and print an old invoice, you will need to change the heading title to what is appropriate for that invoice. If you deal with different situations, you may need to change it on a per invoice basis. Another option would be to create a second business (similar to someone who wants to use multiple currencies) to handle units.
If you want to charge for both on the same invoice, you might need to get creative for a heading title. This column is your multiplier so it has to exist. I haven't tried it, but you may be able to leave the heading blank and use the description to label units or hours.
You change the title by going into PREFERENCES, then INVOICE LAYOUT, then CONTENT. At the top you will see a drop down box for INVOICE TEXT ITEMS. Select the Quantity Column Title and put whatever wording you want in there.
Hope this helps!