My brake warning light came on. The manual says it need brake fluid. Do I just need to add brake fluid? Or does the brake fluid need to be changed?

Check the fluid level in the reservoir first. There is a min-max level and your fluid should be between the 2 marks. If it is low, add the correct type per your owners manual (usually DOT-3). If it is not low, have your braking system checked by a shop to determine the cause. Could be several things to make the light come on and if you are not familiar with brakes, take it to someone who is. A fluid change might be needed if it is still the original in the car. Again, a task for a brake shop if you have never bled brake lines.

Question rather, after putting fluid and leaks out while sitting 5 hrs. ,how bad is that?