Sharpening clippers ain't always possible. Many clippers' cutting edges are coated with diamonds or something else. That hard layer will wear off with use and expose a much softer metal that cannot be sharpened.
You will have to ask the manufacturer or the retailer. High-end manufacturers sometimes have a sharpening service.

Last year I tried to sharpen my Bosch garden clippers. I managed to get a sharp edge on them again but it didn't last long.