0. Download codemirror tarball ('lib/codemirror.js' is not available in git repository anymore):
   wget http://codemirror.net/codemirror.zip && unzip codemirror.zip

1. Prepare files to upload:
   cd UNZIPPED_CODEMIRROR_DIRECTORY/
   rm -r bin/ doc/ index.html demo/ src/ test/ rollup.config.js package.json mode/*/index.html mode/index.html .[a-z]*
   # 54087b03
   mv addon/edit/ addon/cm_edit/ && find -type f | xargs sed -i 's#addon/edit/#addon/cm_edit/#g'
   # https://github.com/codemirror/CodeMirror/issues/5321
   sed -i '/"Ctrl-V"/d' keymap/emacs.js

2. Edit zope.conf to enable FTP access:
   <ftp-server>
    address localhost:2202
   </ftp-server>

3. Upload files to Zope:
   yafc ftp://zope:zope@localhost:2202/erp5/portal_skins/erp5_code_mirror/codemirror/
   yafc> rm -r *
   yafc> put -r *

4. Run portal_alarms/Alarm_checkSkinCacheActive?fixit:int=1