You need to use a database admin tool. PHPMyAdmin is the most popular and many hosts already have it installed. You would then just go to the qa_posts table, Structure view, click edit on the content field, and change where is says 8000 to whatever your value is.
Note that if you have made the field smaller, you don't need to change the database. For example if you set it to 4000 characters, Q2A won't let anything longer than 4000 characters be stored in the database.