Just cancel ("skip") the early ones.
Works, but just be careful because the system sometimes automatically moves them up again!
Delivery dates are fixed for (I believe) the next six months so nothing already assigned a date within that time frame should ever get changed, but if you try to push the first delivery out longer (say, 9 months for example) you can expect the system to automatically re-assign them to earlier slots - right up to the minimum allowed.
If you push them out so the first one is due JUST less that six months from now it should work.
If there are plenty of free slots for the times you want the a/c it may just be best to delay your order for a while.
The only time I've ever tried this was with an empty production line that was about to close. I needed to order right away to keep the line open but I didn't need (and couldn't afford) the aircraft right away.