Turn off the calculator.
Hold down the decimal point.
Continue to hold the decimal point key, while turning on the calculator.
HP 12C Calculator - Changing the Display from a Comma to a Decimal Point
This document describes how to change the display of the decimal point on the HP 12C Calculator from a comma back to a decimal point.
The comma should change back to a decimal point.
The comma is the European equivalent of the decimal point.
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