Sigma camera lenses are made with many elements, each a glass lens itself, but the front lens element is exposed and most susceptible to damage. Removing the front lens with a spanner wrench will allow you to remove the lens locking ring and remove the lens element so that you can inspect, clean, or replace it yourself.
Spray the microfiber cloth lightly with lens cleaning solution and wipe the front lens clean, and allow it to dry completely. Cover the lens element completely with safe release masking tape (This type of masking tape will leave no residue when it's removed).
Find the indentations along the outer locking ring that holds the front lens element in place. The indentations are two small, square notches. Adjust the spanner wrench to fit snugly into the indentations.
Rotate the spanner wrench gently to remove the locking ring. Remove the front lens element by carefully tilting the lens body. Cover the open lens body with a clean, lint-free cloth or microfiber cloth to prevent any dust or dirt from entering.
Wash your hands to remove all body oils and dirt before touching a lens.