You can't, because... there isn't a solution posted.
To be clear, we don't have a thing that marks a question as "solved." We have a thing for marking the accepted solution. These are not the same thing.
If you've solved it yourself, great! Consider sharing the answer with us and accepting it to show us your accepted solution, so that other people in your situation can learn from it.
If you just want to communicate to us "well, I solved it and I don't need your help anymore," there isn't a solution for that. It doesn't matter to us much if you've privately solved it. Delete your question if you're no longer interested in answers. But we do encourage you to post your solution and accept it, if you can.