To install BL do I simply delete Destiny 2 from main menu the night before then next day go to library and download BL then install it, or are there other steps?
No, you don't need to delete your existing D2 install. The BL pre-install that will release within a day of BL's release will take care of the update for you. With that said, there is no issue with you deleting your D2 install, just saying it is not necessary as the update will remove and replace it for you.