You can do this by adding a custom stylesheet (.scss) to the styling folder.
Import it into main and add a custom class to it (for example: ButtonRounded)
Add the following to your custom stylesheet:
To make it more rounded, change the amount (3px).
Actually, there is probably a more optimal solution than the one mentioned before. Just navigate to your _custom-variables.scss file in the theme folder and change the $btn-border-radius value to your preferred value. That should change the border radius on all existing buttons without the need for any additional css or classes.
This is what they look like at the moment, would ideally want them rounded.