There is a case when seller wants to change the carrier that already mapped to particular store in OMS. In such cases, seller needs to perform these steps:

Step 1: Go to Integration module, go to shipping carrier management sub-module, and then click on the edit button of the carrier that seller wants to unmap 

Step 2: Under the 3PL tab, unclick the carrier that seller does not want to map to the carrier. Click 'Update Shipping Method' to complete the action.