Did you try updating it via an upload?

Sounds like you may need to open a case with Seller Support, because according to the Seller Help page on COO, you should be able to change it:

Can I change COO information?

Yes. If you have to correct the COO information for your product, update the COO information accordingly. Refer to the corresponding FAQ on this help page for how to update the COO information for your products.

How do I update the COO for my products in bulk?

Visit the Add Products via Upload page.

Click "Get Product Template" under "Update product details."

Open the template you have just generated and populate four columns:

Column A feed_product_type: provide the product type of the ASIN for which you would like to update COO

Column B item_sku: provide the SKU for which you would like to update COO

update_delete column (column position will vary depending on product type): select PartialUpdate (this setting ensures that the values in your catalog remain unchanged for attributes you are not populating in your template) country_of_origin column (column position will vary depending on product type): provide the COO for your product

Save a copy of the file

Visit the Upload Your Inventory File page.

Browse to find your file or drag and drop it, then click upload.

Providing Country of Origin information for your products

After you open your case, post the case ID here in case a Community Manager happens by to offer assistance.