How I can fix my Victa Quatro (Briggs & Straton) Lawn mowers carburetor?

My Law lawn mower does not start, if it starts with lot of effort it runs for 1 meniute and then stops. I have replaced the spark plug and clean the air filter, change it oil etc but still no change. I believ there is probleme in the carburetor, which needs service and clean up. I do not know, how to clean it and what to look for to replace in the carburetor.