You can't edit the banner that announces a new version update, but that banner only shows up for certain users and only once per release, if dismissed it will not show for that user again until the next release. The screens below show how you can create custom banners that can be displayed to users on the instance
Access the setup in Configuration Reference Libraries:
Here you can add new items and set their type to 'Banner':
The Banner Content input here is what will show to users in the banner.