Joomla's language ovrride feature enables you to translate/change a text without editing the language files.
Here is a short guide specific to K2Store.
Let us assume that you want to change the Firstname (in billing address fields) to My Name.
- Go to Joomla admin->Extensions Manager->Language Manager->Overrides.
- Click new to create a override
- Under the Search text you want to change, enter the following text (as you see in the site): Firstname and hit search.
- You will get the constant: K2STORE_FIRST_NAME
- Enter this constant in Language Constant * box
- Enter your value (in our example My Name) in the Text box.
You can create as many overrides as you like in this method.
You can learn more about language override from this page: http://www.inmotionhosting.com/support/edu/joomla-25/language/add-new-language-override