On your profile, you can find "edit" option on the top right side on the screen you can add information in there that will appear in the "about section"!
If you're using a browser to navigate the board, just click on your account name in the upper-right-hand of the page and when the menu appears as a dropdown just click on your account name again. That should bring you to https://forums.stardewvalley.net/account/ which is where you should be able to edit things.