There is a button to the right of the password box titled 'options'. From here you can select which build to use. If you have selected 'Always use development builds' or 'Always use recommended builds' then the launcher will update your version when there is a new dev build or new recommended build respectively. If memory serves me correctly the launcher default is to always use the recommended build. If you don't want to auto update, or you want to update before it is recommended, then you'll need to select 'Manual build selection'.