At any time, you can change the padding values, either by entering different values into the property inspector, or directly on the canvas. To edit padding on the canvas, press and hold the ~ key (top left of your keyboard) and hover over the top, left, right or bottom padding areas. Once you see the pink overlay you can click and drag to resize the padding.
If your buttons also include icons or other elements like a caret or arrow (for dropdown buttons), using a Stack in your button can take it to the next level, and make it very easy to swap arrangement of the button content, and control the spacing between elements.
Start by selecting the text and icon and make a group with these two elements. In the property inspector on the right, enable Stack. Adobe XD will automatically detect the direction of the stack based on the element positioning.