You'll have to ask the template developer, or at least show us the list of layers in the template. Just find all the text layers, and one of them should be the one. Sometimes templates come with text that cannot be changed, which makes them useless as templates. If you find that is the case, just create your own text layer, and use the same coordinates, animation keyframes, and parameters as was used for the other layer, and you should be good to go.
Two things to try. You said you know what layer it is, or think you know. If you double click the layer and it IS a text layer, you'll be able to type the new text in. If it really isn't editable, create you're own text, and if you have cs6, parent the new text layer to the layer you want to replace, hold shift before you let go of the pick whip and it will snap to that layers location, thus eliminating any need for copy pasting keyframes. Just increase the scale to whatever size you need it, and you're good to go. Don't forget to make the old layer invisible of course.