Since you have not given us the model number of the mower this will have to be a generic answer.
Do these steps in order or do not waste your time.
Clean your fuel lines and fuel tank of all old gas. Blow out with air when gas is removed. You can filter that gas and use in your car. Cars are more tollarant to bad gas.
Replace fuel filter.
Clean carb.
Dirty carb
Down and dirty: https://www.youtube.com/watch?v=VBL-BUj0c6E
More technical: https://www.youtube.com/watch?v=nAk_WDt3AD8