Given the seemingly high interest I'm considering researching a full post on this. However, going just off what I remember from psyc classes the short answer is we don't know and suspect that you can't alter them much, at least not without doing something drastic. That said, putting yourself in social groups were the desired trait is common/expected looks relatively promising. Additionally, it may be possible to change your behavior in ways that emulate the effects of a big five change without technically changing your personality. In particular the Getting Things Done I learned from CfAR is supposed to boost your effective conscientiousness, and I have observed that to kinda be the case since I adopted it.