Cleaning data

Hello Sir, Could you please clear my doubts :- -How to decide whether outliers should be removed or replaced or left in the data . suppose in budget column there are values starting from 218 to 12215500000. -some cells are blank in director name column .can I fill those column with ‘unknown’ or leave as it is ?How do we know what to do with those blank cells in a data .Does company specify anything ? Thank you