hi, I have upgraded my machine to beowulf. Thanks to all the people for their instructions. My steps.

# apt-get update && apt-get upgrade
Replaced all -ascii- to -beowulf- in /etc/apt/sources.list
Created /etc/apt/preferences.d/avoid_some_beo like fsmithred did.
# apt-get update
# apt-get upgrade
# apt-get dist-upgrade
# reboot

Issues:
firefox 67.0b2 tab crashes while playing any video
no audio in youtube video using vivaldi browser instead of firefox.
fish shell does not work.

Comment out autospawn in /etc/pulse/client.conf.d/00-disable-autospawn.conf fix firefox issues.
uninstall fish
purge fish configuration files and omf configuration files
install fish