Change 'Older posts' and 'Newer posts' to text of my own choosing

I’m using Theme Twenty Ten.
When I select a category or the ‘Swim in The Pool’ page, the navigation is simply to move to ‘older posts’ or ‘newer posts’.
I would like to change this to text of my own choosing. Please help me out, i’m not sure how to customize the css let alone where to place any customization code. Thanks for your time

http://www.photography-pool.com/swim-in-the-pool/