Root user on pi. Need to edit convert.conf file

Hi

After switching to LMS on pi I got my old problem with stuttering sound on some tracks from Tidal back.
I got rid of the problem by editing convert.conf file with this:

flc flc * *
[SOX] -t flac $FILE$ -t flac -

But I can only log into my pi with the tc user, and it is not allowed to change anything.
The pi login does not work with standard password raspberry ??? I have not changed it...... ???

I don't know the root password ?????

Any ideas what to do ???