If you have installed packages from testing in Devuan stable and want to revert your system to a state in which it only contains packages from the stable branch, follow these steps.

1. Login as root, go to /etc/apt/prefrences.d/ and create a file named "force-stable" (or name it anything you like) there:

# cd /etc/apt/preferences.d/
# touch force-stable

Now, put the following lines into that file:

Package: *
Pin: release a=stable
Pin-Priority: 1001

Pinning the priority of stable to a value beyond 1000 will only allow packages from the stable branch to be installed on your system. Any packages from elsewhere, including backports and security upgrades (which come from a dedicated Devuan repository of their own), will be downgraded.

2. Upgrade your system:

# apt-get upgrade

In case this renders any packages obsolete, uninstall them with:

# apt-get autoremove --purge

3. Delete the file you created in /etc/apt/preferences.d/ or move it to another location, if you want to save it:

# rm /etc/apt/preferences.d/force-stable

or

# mv /etc/apt/preferences.d/force-stable /home/myself/good_spot

4. Deactivate the testing repositoriy in your /etc/apt/sources.list:

#deb http://de.mirror.devuan.org/merged/ ascii main
#deb-src http://de.mirror.devuan.org/merged/ ascii main

5. Update, re-upgrade, clean up

Now run

# apt-get update

to make apt aware of the new configuration.

As all security upgrades were lost in the downgrade above, you need to bring your system up to date by running

# apt-get upgrade 

once again.

Finally (and optionally), wipe obsolete packages from your disk with

# apt-get autoclean