If the window frame itself is warped, or if the wall around the window has crumbling bricks, you need a builder or window fitter, not just a tube of silicone.
We get asked this question all the time: "Can you reseal uPVC windows?" can you reseal upvc windows
Renewing the silicone caulk between the window frame and the brickwork/wall. 🛠️ Step-by-Step Guide to Resealing 1. Replacing Rubber Gaskets How to Replace a uPVC Window Seal — DIY Guide (UK) If the window frame itself is warped, or