How can I increase the upload border radius?

On the upload theme style editor, the rounded border can be increased up to 15px:

Larger border radius

The max border radius configurable in the app is 15px, this limitation is due to the way the button is structured.  However, it is still possible to increase the border radius by applying a custom style for the full upload area,

This can be done in the Shopify Admin theme editor, navigate to Online Store > Themes,  [More actions] > Edit Code.

Open the file theme.liquid:

There you can add the following code into the head section:

<style>
.cl-upload--area {
   border-radius: 50px;
   overflow: hidden;
}
</style>

You can adjust the border-radius value to fit your themes style. 

If you need further assistance, please send us a request and we will be happy to help you out.