I did it for you. For future reference, you will have to set the topic as a question first, then you can change it later to solved. On the bottom right of the posts, there are three dots in a vertical line. Click on that to change the topic to a question (which will mark it 'Unsolved'), then when you’re ready to change it, click on the three dots again and you’ll have an option to set it to 'Solved'.