To replace the Background Sheet (template) in a Solid Edge Drawing (Draft), ensure you can see the Background sheets (if not, right click on a sheet and toggle Background on). Right-Click on the Background sheet to replace, and choose Replace Background, then Browse to the .dft file that contains the desired background, ensuring that you choose Replace blocks of the same name if the new background template contains blocks with the same name as the current background template.
Click OK and your Background will be replaced.