Extensions to Existing Visas
Before your existing visa expires, you must apply. Within 28 days of the expiration of your current visa, your new course must start. If your visa, for instance, expires on October 1st, you must apply for a new visa before that date. By October 29th, you must start your new course.
The type of UK visa you have will determine whether you can extend it and how.
If you continue to meet the requirements for visa eligibility. In any case, you must make your application well in advance of the expiration of your current visa to avoid overstaying, and you must pay all applicable fees, including the Immigration Health Surcharge.