Leaking arched windows is not an unusual problem. The arched shape of the windows means that flashing is typically not installed between the window and the wall frame (like with a regular rectangle window) and only caulk was used to provide the weatherproof seal. Depending on your climate, that caulk my only last a year or two before needing to be replaced. Recaulking will fix the leaking arched windows, the hardest part of the job is getting up on a ladder to reach it.
Things You'll Need
Ladder (if needed)
Climb up to the arched window. You want to be able to reach and work on the frame surrounding the window without having to move your ladder (if you are using one) too much so try to pick a position in which you can reach as much of the window safely as possible.
Cut into the old window caulk with a box knife while holding the blade of the knife against the window frame. Slide the knife around the whole window frame, using it as a guide by keeping one side of the blade pressed to it. You are cutting the seal of the caulk off the frame, which is why you want to keep the blade against the frame.
Pry the old caulk from between the arched window frame and the house.
Cut the tip off your tube of silicone caulk. Most caulk guns have a circular hole in the side of the handle that if you watch while you squeeze the trigger, acts like a small guillotine. Stick the tip of the tube in the hole and squeeze the trigger to cut it off. Then poke a hole inside the tip of the tube of caulk, using the metal rod attached to the caulk gun. To do so, just poke it straight down the tip and you will feel it break through. Load the tube into the caulk gun by releasing the back and pulling the push rod as far out as you can. Put the tube in the gun and squeeze the trigger several times until the end of the push rod is against the bottom of the tube.
Recaulk the arched window. Start at any given point around the window frame, stick the tip of the caulk into the space between the frames. And holding the gun at a 45-degree angle, steadily squeeze the trigger while moving the gun along the frame. Don't worry if it doesn't look good now, as long as every time you have restart your caulk line you are overlapping the beginning and ends of the caulk it will be fine.
Drag your finger tip at a 45-degree angle along the top of the caulk to shape it and smooth it into place. Use damp rag to clean up any excess before it dries.
If you have to move your ladder to caulk the entire window, finish the area you are working in completely, including shaping and smoothing so you do not have to come back to it.
Working from ladders is extremely dangerous. Make sure that your ladder is in good working condition, that the feet are stable on the ground and the angle is such that it will remain in place against the house while you are recaulking. If at all possible, have a helper hold the bottom of the ladder while you work.