These words are not interchangeable in standard speech and writing. Easily is an adverb; easy is an adjective. Such expressions as "Take it easy," "Go easy," and "Travel easy by bus" are informal, although widely used. Examples of standard use: "This is an easy task." "You can do that easily." Because they are trite or highly informal, avoid such expression as "Easy does it," "go easy on," "easier said than done," "slow and easy," "easy money," and "on easy street."
Dictionary of problem words and expressions. Harry Shaw. 1975.