"Section Headers" are summary tasks and most of their attributes (such as
work and duration) are calculated read-only values based on their
subordinate subtasks. To change a summary, you have to change its subtasks.
Consider whether it's even going to be physically possible to condense the
timeline as you wish. IMHO, one of the great fallacies of project
management is that you can arbitrarily pick a timeframe in which to do the
project and expect it to actually happen that way, the belief that
will-power is the determining factor. But the reality is that many, if not
most, of the tasks in a project have a maximum rate beyond which they simply
cannot progress. As an example, if you have 1 paint sprayer that lays down
10 square feet per hour and 400 square feet of wall to paint, it will take
you at least 40 hours to complete the task no matter how badly you want it
done sooner and there's not a thing you can do about unless you can either
get another sprayer or upgrade to a faster one. You can't base your
schedule on how soon you want it, you have to base it on how long it's going
to take. It could be that the minimum individual task durations and the
physically mandatory sequence in which they must be performed simply make it
impossible for the project to be completed in 6 months
