You can change the button shape in two ways:
Global button shape settings
Expand the Website styles panel on the left, open the Buttons tab, and select the preferred button shape (oval, rounded, rectangular):
If you make changes in the global style settings, they are applied to all buttons across your website, including regular buttons, “Add to bag” buttons (single product and product list sections) and submit buttons (contact and subscription forms).
Local button shape settings
Within the editor, click on a button element and select Edit button. In the settings window, open the Style tab, and change the corner radius:
Contact and subscription forms: Submit button
Click on the contact form, and select Edit form. Open the Style tab, and select Submit button from the dropdown. Finally, change the corner radius: