Step 1:

Upload your design here, confirm you dimensions, pick a material, and add to cart.

Step 2:

Rather than proceeding to checkout, exit the cart by clicking the little 'x' at the top right

Step 3:

Click on 'design' at the top left of the page, then on my designs to bring up your files:

Step 4:

Select your file and update your material (it will have defaulted to the last one used).

Step 5:

Add to cart again:

You can proceed to checkout from here, or add more parts.