To make that change, you would need to have access to the source code and modify the text of the links... you cannot do that on a wordpress.com hosted blog. That said, and seeing that you have the CSS upgrade, this kludge might be helpful. Add this to your CSS:
.nav-above {overflow: hidden;}
.nav-above .alignleft a, .nav-above .alignright a {text-indent: -9999px; display: inline-block; width: 100px; position: relative; z-index: 10;}
.nav-above .alignleft:after{content: "Previous"; position: relative; left: -100px; z-index: 1;}
.nav-above .alignright:before{content: "Next"; position: relative; right: -100px; z-index: 1;}