You can retake your GCSEs at any stage of your life or career. The only thing is, later on down the line, you will probably have to pay to do so, unless an employer is going to fund the course and exams for you. If you have just finished school or college and want to retake your exams, or you are a teacher with pupils who want to do this, it is going to be the easiest and cheapest option.
You may be able to retake exams in a school or college environment. Speak to your institution to see what they are able to offer you. If it is not possible there, you may be able to take the course somewhere else locally.
As an adult learner, look into adult learning courses at your local college. You might also be able to take some courses online and just sit the exams in person.