First, it's highly unlikely to be damaged by feeding it 12v instead of 10v but if you want to reduce the voltage, there are two easy ways.
First, feed the 12v through 3 diodes (Part No 1N4148) connected in series; each diode develops 0.6 - 0.7v across it so you are left with about 10v at the output.
Second, use two resistors as in the previous reply, connected as shown. I've given you suggested component values (180 and 910 ohms) to use which will reduce the voltage from 12 to 10.
Any dealer who can solder should be able to do this for you.
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