There is not a way to modify App User ID’s directly in the dashboard.
An App User ID will always be assigned to a user of the app when the SDK is configured. If you do not configure the SDK with a provided ‘custom’ App User ID, then the SDK will automatically assign the app user an Anonymous App User ID.
The way to “assign the app user ID” is when the SDK is initially configured, or by calling .login() after the SDK is configured. Depending on your project settings, this may cause an alias to occur - essentially merging the App User IDs together to act as one.
Could you let us know more about the use case, and why “it cannot be assigned in the app for subscription information”?