I'm new here. I have posted a question here Question about n-simplex and its face
And it has been solved, but I don't now how to mark a question indicating that it has been solved.
The only mechanism to mark a question as "solved" is for you to accept an answer. This is because Stack Exchange exists as a Q&A site, so the only reason we'd want to mark that a question has been adequately handled is that it's been given an adequate answer.
In the given case, since no one has written an answer, I would recommend either that you write an answer yourself or encourage someone who left a helpful comment to do so. Then, when the question has an answer, you can accept it.