If you can’t find a master slide that closely resembles the master you want, you can create one from scratch.
In the master slide navigator, select a blank slide.
If the theme you’re using doesn’t have a blank slide, select a master slide, choose Edit > Select All, and then press Delete.
Before you modify a master slide, you might want to duplicate it so that you can continue using it. See Duplicating a Master Slide for instructions.