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.