It may not have been setup with the programming for name change. Just use the normal instructions for changing the PLU or department (whichever you have) name. It will be in your manual.
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If anyone needs a Geller / Towa ET-6600 manual, C-Tech has the full operating and programming manual here:
http://the-checkout-tech.com/manuals/cashregisters/Towa_ET-6600_and_Geller_ET-6600_operating_and_programming_manual.html
Registerman
February 2011