Because PowerPoint doesn’t use styles, it’s not obvious exactly how the formatting gets applied and how you control it.
The first thing to know is that you simply can’t control the underlining, it’s there and it’s there to stay. You can, however, change the colors used.
Click the Design tab > Colors > Create New Theme Colors, locate the Hyperlink color and change it to something different if desired. To control the Followed Hyperlink color, select that option and change it.
You now need to save your changes as a custom color scheme, so type a name for your color scheme and click Save.
Now the chosen hyperlink color and followed hyperlink color will be used to format the hyperlinks in your PowerPoint presentation.