Changing end date on progressed activity

I've just started using Asta PP after working on P6, and I'm a bit thrown by how you accomplish even simple things.

In P6 once I had marked the progress on an activity you could use the Anticipated end date to change the planned end date of a task, now this may be a dense question, but how do I do the equivilent in Asta?

If I try and change the finish date, it simply jumps back to the original finish, it won't accept the new date I'm giving it.

Thanks

h-angell