Try this:
1. Select the slide to which you want to apply a new template to, by clicking on your preferred slide in the Slides "Pane", on the left side in PowerPoint.
2. Select the design template from Format > Slide Design and choose the new template to apply to your slide by clicking on the template drop-down menu (right click a design template that appears on the right) and selecting the option "Apply to selected slides".
Source: this