Once you have the fonts used in the template installed (click here if you need help with font installation), open the PSD template file in Adobe Photoshop.
Select the Text tool from the main Photoshop menu.
On the Layers menu (if you do not have this enabled, hover your mouse to the top bar: Windows > Layers and it should appear), select on the text layer you want to edit.
Type your desired text in.
If you wish to change the font, click to the font menu to select a new one. If you would like to change the font color, click the color icon on top.