You're left wishing
Once you mark a post there is no way to go back. You can add a post explaining the situation and what solution you actually found.
If you're concerned that people may bypass your question since it appears to be solved, you can start a new thread, explaining that you inadvertently marked the issue as "solved" in the original thread, and including a link pointing back to the original question so people can see what solutions have already been provided. I'd recommend also including a brief description of the problem so that people don't have to read the original thread to know if they might have information that would be relevant.