Restoring previously saved environment

None of the previous saves are deleted (unless you explicitly do that). All save files are kept in ~/.tmux/resurrect/ directory.
Here are the steps to restore to a previous point in time:

make sure you start this with a "fresh" tmux instance
$ cd ~/.tmux/resurrect/
locate the save file you'd like to use for restore (file names have a timestamp)
symlink the last file to the desired save file: $ ln -sf <file_name> last
do a restore with tmux-resurrect key: prefix + Ctrl-r

You should now be restored to the time when <file_name> save happened.