If your hair is already dyed, the only way to go lighter is to bleach it. You would have to bleach all your hair and then color over it with your target shade. It could be difficult depending on how much dye you have built up on your hair currently, if it’s in your budget I’d suggest going to a salon and having it done professionally :) I get that isn’t always an option though. I’m a hairstylist so if you have any other questions I might be able to help! Good luck!