How to edit custom-convert.conf of LMS 8.0.0 on Synology DS116 with DSM 6.2.3. ?

Newbe question I suppose: How to edit the custom-convert.conf of LMS 8.0.0 on a Synology DS116 NAS with DSM 6.2.3. ? If this is possible... What is the right folder, how to get there and find and edit or replace the conf-file. And change it back if it doesn't do what I want.
Thanks!