Does the caulking need replacing on your windows?
When you have new windows fitted, there will usually be a rim of caulking applied around the window edge to create an air tight seal. Over time this caulking can start to come away and may need to be replaced. When it comes to windows, there is often very little you can do your self. You may need to hire the help of a professional as if you make a mistake and damage the glass or the trim you may be forking out hundreds of pounds if not thousands for a replacement.
You should also apply caulk to the inside of the windows too to help protect it. This will seal any unwanted air leaks. Using a caulk gun will ensure you fill any gaps and get a clean line. It is often quite hard to get it exactly right, so you may want to run a wet finger over the top of it to push it in to any cracks and ensure you get a smooth finish. Every few years you should try and have the windows redone to ensure that they last as long as possible and that you do not have to replace them in the near future.