Download the latest version from here: http://www.question2answer.org/versions.php
Then unzip the files and copy them to your Q2A directory, i.e. overwrite what you have. Then upload to your server.
It won't overwrite custom themes and plugins you have but if you've edited anything in the core you'd need to make those changes again. See here for some tips about upgrading when you've midified the core: http://www.question2answer.org/qa/8315/overwrite-functions-without-creating-problem-version-released
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.