Hi,
Are you looking to just change the text or make a link that will take you to the previous page?
Either way, if you are trying to just change the text you will need to do a language override. Go to your language folder, open it and you should see an override folder inside that you will see a override file. Add: K2_BACK_TO_TOP="back to previous" to that file .
As far as giving the link the ability to go to previous page I'm not sure how to write that in php but you could try the following,
Replace this:
Joomla! v3.5.1
Gantry5 - Hydrogen Template
k2 v2.7.0