If I see a question that has an answer and it's been already accepted, should/can I answer? What is the site policy about this?
You can post it and if you think your answer might be better, more up-to-date then you should.
Don't let yourself discourage by the accepted answer. That can be changed. If you provide a new answer to an old question, which has more detail, it's better formatted, it provides sources, cites references and explains clearly the answer, then it's likely that the OP (Original Poster) will change the accepted answer and choose yours.
In some cases, this might even bring a bounty to your answer, so what do you have to lose? Since you don't need to hurry, work on your answer, fix it, improve the wording and then submit it!