Conda keeps a history of all the changes made to your environment, so you can easily "roll back" to a previous version.
To list the history of each change to the current environment:
conda list --revisions
To restore environment to a previous revision:
conda install --revision=REVNUM
conda install --rev REVNUM
NOTE: Replace REVNUM with the revision number.
If you want to restore your environment to revision 8:
conda install --rev 8
NOTE: Use Anaconda Prompt or Terminal window to run these commands.