How to cancel requesting or remove custom alias of ECR public registry?

By default alias is assigned to your public registry after creating your first public repository. A custom alias can be requested in the public registry settings in the Amazon ECR console.

yes, its possible to change/update the custom alias but you have to wait for approval from AWS before you can change it ,please refer to the links for guideline[1]

please consider reaching out to AWS Support[2] for further assistance

Reference links

[1] AWS Documentation-https://docs.amazonaws.cn/en_us/AmazonECR/latest/public/public-registry-settings.html

[2] AWS support - https://aws.amazon.com/contact-us/