I am in year 12 and I was in the exact same position as you. I was taking maths, physics and computer science. After looking at entry requirements, I saw that further maths was much more useful then computer science so I changed. I only changed about 2 weeks and have just finished catching up today. It’s not too late to change to further maths. To help with catching up if you do choose to take further maths, tlmaths on YouTube is great and explains content very well.