Will a score of 15 on the ACT over ride fcat test?

No, an ACT score of 15 will not "override" an FCAT score. The FCAT (Florida Comprehensive Assessment Test) and the ACT are different tests used for different purposes. The FCAT was primarily used for Florida state accountability and student progress within the Florida school system. The ACT is a national college admissions test.

They are not interchangeable or used to replace each other. Colleges look at ACT scores, not FCAT scores, for admissions purposes. Florida's high school graduation requirements may have once involved FCAT scores, but that system has largely been replaced. Your high school and/or college will have specific requirements you need to meet.