SHIFT Click on Facilities erases them and the links attached to them.Select an Extractor to change the head number.Don't forget to set your Skill level and the Command Center upgrade.Don't forget to set the planet radius so that links are as close as possible from in-game values.
DISCLAIMER :This is not maint to be a simulator, only a Setup Builder.The goal is to check if it could fit on said planet, with given skills.If you want a true in-depth simulator, you can find several in the Third Party Devs section on the Official Forums.