You can stay in the Schengen area for a maximum of 90 days in any 180 (the 90/180 rule1). If you've already been in the area for 90 days you must stay away for 90 days before you return.
Be aware that if you return after 90 days, planning to stay for another 90 days, sooner or later you will be denied entry as it will appear that you are trying to live in the Schengen area through repeated visits.
There is no way to extend your stay under the 90/180 rule. If you wish to stay longer you must apply for a national visa D that will permit a stay of up to six months. More than that will require a residence permit.
1Codified in Regulation (EU) 610/2013