You might need to change the mark-up on your products after the products are already added to Shopify.
While Spocket does not have control over the prices on your store once the products are pushed, we can suggest a few ways you can go about it.
You can go to Shopify and bulk edit the prices of the products. You will not have the option to use multipliers or percentage markups, however. Here is a link that will help you understand how bulk edit the prices: Bulk editing on Shopify
If you wish to increase or decrease prices by percentage, you can try this app: Bulk Product Edit
If you wish to check the products that you have pushed to Shopify through Spocket, you can view the My Products page.