Hi Alberto. It depends on the brand and model. Some can only be set when fully cleared of programming. Sometimes the battery is the problem. Please let me know the brand and model.
-----------------------------------------------
If anyone else is having trouble, C-tech has a huge range of cash register manuals here:
http://the-checkout-tech.com/
Registerman, March 2011
edit
I can answer this question...
we can not erase the "coma".showing purchases in decimals?