I've been watching Susskind's lectures on string theory and M-theory. He mentioned in video 3 that string theory initially had tachyons but that this was fixed early on in string theory's development. I'm wondering how precisely one can get rid of tachyonic fields (fields with negative mass squared) in string theory. I see from other questions that increasing the dimensions may somehow get rid of the tachyons. If so, how? What would be some good sources (such as review articles or videos) for me to learn more about how one can remove tachyons from a theory?