We cannot know without seeing the structure of the project. Provide screenshots of the timelien and project window or a link to where you bought the template from. Most likely it's simply somewhere in a nested comp. Anyway, revisit the instructions/ tutorial/ demo video that comes with the template. Usualyl this is pretty clear...
Text layers are identified with a T in the timeline:
If you don't see a T then you'll probably see a Nested Composition:
If you don't see either, then it's a footage item.
To edit a text layer simply double click it in the timeline and All of the text in that layer will become selected and the type tool will be active. Type in your new text.
If your text is in a pre-comp or a nested composition then a double click should open the nested composition. Look for the text layer in that comp and double click in the timeline to edit. Templates usually use nested compositions for text. That's probably your problem.
If it's a footage layer you need to edit then you select the footage in the Project panel or the Timeline and select Edit>Edit Original from the menu. This brings up the program used to create the footage and you edit there. This is not usually the case with Templates.
If you can't figure it out from these suggestions, and the template didn't come with instructions, contact the person that sold you the template and have them tell you how to use it or get your money back.