As with most Joomla! modules, our styling of the module layout doesn't suit every website. You are free to customise the layout and styling as you please. We recommend that if you are going to do any customisation, that you following the Joomla! Layout Override procedure and leave the core code untouched.
There are many articles available on how to use Layout Overrides in Joomla!. I am not going to rehash the process, other than to say the files must be stored in the templates/{your template}/html/mod_k2_split_content
folder.
Once you have created a Layout Override, you can select it from the Advanced tab of the K2 Split Content module configuration screen, and it will be used to override the standard layout supplied with the module.