If you have not downloaded it yet, then click on 'add ons' and then 'add'. Scroll down to the 2015 BJCP Guidelines and highlight it. Then click 'install add-on' and it will download onto your beersmith folder.
Next or if you have already d/l'd the add-on, click 'options', then where it says 'Display Style Guideline', change it to the 2015 BJCP guidelines and they will now be reflected in any NEW recipes you create. To change the style guideline in an existing recipe, open that recipe and click on the style drop down menu. The 2015 guidelines will now be there for you to update that recipe to the new style guideline.