This document will link you straight to each sales channel and market places help guides on how to bulk update your SKU codes. 

Amazon: 

How to edit listing and product information

Build your inventory file

How to use the new manage inventory

eBay:

Tip: eBay suggests for merchants with SKU codes to use there Merchant integration platform -  a bulk management tool for high-volume sellers

File exchange and Merchant Integration Platform info

Merchant integration Platform

eBay selling manager pro custom label information

Shopify:

Bulk editing products and variants

Woocommerce:

Adding and Managing Products

Bulk updating via CSV export and import 

Magento:

Tip: Magento offers an add-on to bulk update your product information, including SKU code.

Magento bulk product update

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