You can easily edit a release date once it's already been distributed.Please note that while an existing distribution is processing, you can't make changes for those specific DSPs. It's best to wait until they're processed. 

Here's where you can edit the release date:

  1. Click on Assets > Music
  2. Click on the asset you'd like to edit the release date of.
  3. Select "distribution" from the menu on the sidebar (see image below)
  4. Click the big purple Distribute All button on the top right corner of your page
  5. Scroll to the bottom to find the "Release start date" field and edit it there.