Open it up and clean it. I haven't opened this particular brand of lens, but basically they are all the same - and different. You'll need rubber grips to get the name ring off. Behind that there is probably the front lens barrel. You will most likely have an option of unscrewing a thin ring holding the frontmost element in place, or screwing off the whole front lens barrel. I would screw off the barrel, clean the outermost lens surfaces and see if the barrel itself needs to be opened. If you need to open it up, you'll end up with the front element in your hand which is a bit challenging to get really cleaned up as you need to hold it as you clean it. Microfiber gloves are great here as they don't leave fingerprints or lint on the surfaces.
From rear side, it's quite similar. The lens mount is usually attached by screws. If this lens has an automatic aperture, the actuator mechanism is in there, too. Take note of how it was put together, so you can reassemble it correctly.
For cleaning, I use different solutions depending on the particular kind of dirt. I have 1:200 Ilford Ilfotol, industrial alcohol, lighter fluid and for stubborn haze and fungus I have vinegar. Use an air duster to get loose stuff off the lens, and be very careful about not accidentally rubbing hard particles against the lens surface. You can imagine what happens if there is a grain of sand between your q-tip and the lens.
If the aperture blades have oil in them, use lighter fluid to clean it off. You may or may not need to disassemble the aperture to have it all come off.