Change end date of a multi groups projects

Have an already seated project made of multiple groups of task and I want to change the end date (as long as the dead line has been moved forward).

I’d like to change the end date from the main group and having all the sub-groups to adjust proportionally with the extra time I get. Is that possible?

It’s odd that I cannot even change the date of the main group: if I select a different date nothing change at all.

Thanks in advance!