Title has been released but won't let me change release date

I assume you are talking about the TV Series  Shimmer and Shine (2015).

You cannot update most things on the Series page. The correct method is to update the  episode page for the first Episode, since after all it was actually the Episode that premiered. The system should then copy that date up to the Series page, replacing the date already there.

This was actually explained in the error message when you tried to update the series Release Date :

This credit may only be added at the episode level
You appear to be adding this at the parent TV-series level and this is no longer supported. Please specify the exact episode(s) to which it applies.

You could have  clicked the listicon at the bottom right of the Episode(s) box, and selected the correct episode, or you can go to the episode in question and enter it there. In fact it looks as though someone did add the date to Episode 1.2; was it also shown that day, or is this an error?

Note that the update to the series page may not occur for several days after the Episode is corrected.