This is for Joomla 1.5 version although it should be same for Joomla 1.7 Version.
Goto
1. Joomla Admin
2. Components
3. K2 Store
4. While you are on dashboard look on right you will see Parameters click on it. You will get a popup window for K2 Store options.
5. Scroll down until you see " Date, Currency, Number Settings "
6. Find Currency Symbol and Currency Code and change them.
See you around...