The present perfect (which you used in the first sentence) is much better. This is because we use this tense for something that happened in the past but which is very relevant to the here and now (in the present) and the immediate future. In this case we would say: "The release has been postponed for 2 days." "To postpone" means to delay something until a future date. This is a better and quicker way then saying "the app hasn't been released yet," and then saying that it has been delayed and telling people the new release date, because using the word "postpone," tells the reader both that a delay has happened and that the release has been re-arranged.

Hello Rafaela, You should use 'The app hasn't been released yet, they have delayed the release for 2 days. After that it will be available for download ...' This is the present perfect as it talks about an action (or inaction) in the past which has an effect in the present. Best wishes Bob