So first you have to backup your original wallet. Then, move it out of the data directory and update Bitcoin Core. When you start Bitcoin Core 0.13.0, it will generate a new HD wallet for you. Grab a few addresses from that wallet and write them down somewhere. Then stop Bitcoin Core and backup the new wallet.dat file. Replace the wallet.dat file in your data directory with your old wallet. Restart Bitcoin Core, it will load the old wallet and you will see that it is non-HD. Send Bitcoin from your old wallet to the addresses that you copied down. The Bitcoin will go to the new HD wallet. Then stop Bitcoin Core again, replace the wallet.dat file in the data directory with a backup of the new one, restart Bitcoin Core, and you will be good to go.
Ok this is confusing, all I've ever done when updating/upgrading bitcoin core is download it off bitcoin.org
My coins & old addresses are always there when I open bitcoin core after updating/upgrading it.
Are you saying that with the 0.13.0 update that isn't the case.
Obviously I have a back up of my wallet but speak to me like I'm 10 years old.