When you press the [M/EX] key, it toggles between conversion mode and memory mode. You will see "EXCH" on the display when in conversion mode. C1, C2, and C3 are three conversion rates. C1 is your home country currency, and so it is always set to 1. C2 and C3 are rates for two other countries, which you can change. For example, if you want to set a conversion rate of $1 to 0.95 euros, here is what you would do (using C1 for US dollars and C2 for euros):
1. Press [AC]
2. Hold [SET/%] for about two seconds until "SET" appears
3. Press [C2] to select currency 2
4. Input the conversion rate (0.95) and then press [SET/%] to store it in memory
You can check the currently set rate at any time by pressing [AC] and then [C2].