To start from scratch either pick up a new blank SD to flash an image on, or reuse the existing one, wiping the current content in the process.
I use OSMC for my own media center and am quite happy with it, it's quite straightforward IMO. You can download it from osmc.tv
Openelec is another popular ready-to-go media center solution. Alternatively you should be able to install and run Kodi on top of Raspbian. If you only need a basic DLNA server, minidlna, available from the raspbian repo, is pretty good, but setting it up properly will require a bit of work.
Ultimately what's best for you depends on your needs, the latter option being the most flexible if you wish to put your Pi 2 to other uses.