The short answer
Warm water with a little washing-up liquid, a soft brush for the frames, and a squeegee for the glass. Nothing abrasive on uPVC, which scratches and then yellows.
- Brush the frames and sills first with a soft brush and soapy water.
- Wash the glass with a little washing-up liquid in warm water.
- Squeegee off in overlapping strokes, wiping the blade each pass.
- Dry the edges with a cloth.
- For high windows, a water-fed extending brush is far safer than a ladder and gives a better result.
What will ruin it
Never use cream cleaner, wire wool or a magic eraser on uPVC frames. They are mildly abrasive, and the micro-scratches they leave hold dirt and discolour permanently. There is no way back from that except replacement or specialist restoration.
Why it works
uPVC has a thin smooth outer skin over a more porous core. Once that surface is scored the frame is permanently more difficult to clean and it yellows faster in ultraviolet light.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.