Hi there!
If you want to change something that K2 displays by default first look up the language file located in Joomla! directory /language/en-GB/en-GB.com_k2.ini
Be aware of encoding if you are using a different language.
For example if you want to change the displayed by default 'Children Categories', search in language file the tag
CHILDREN CATEGORIES=Children categories
and change it for example
CHILDREN CATEGORIES=Products
You can do the same thing for all text displayed by default from K2.
I think this a better appoach than changing the .php files
Hope this reply helped you!!
George