良い can be read as both いい and よい. 良{よ}い is more formal than いい. But they are very similar words, and they are sometimes safely interchangeable. For example, the following words are the same and both mean "good boy/girl".
いい子{こ}
良{よ}い子{こ}
Sometimes, いい cannot be replaced with 良い in casual language. For instance, in the Japanese version of Facebook, you call the "Like" button, the "いいね" button. If you replace it to "良いね", it would be a little bit strange because いいね is a casual word, and 良い is somewhat formal. They should go together.