Basically, how do I change the default format in linked.json, from:

{
"player UUID": "discord ID"
}

to:

{
"player username": "discord ID"
}

This is a question, and if there's anything regarding this in the config.yml, please let me know

You can't. The entire point of using UUIDs is that if someone changes their name their account stays linked. You're probably running offline mode or something but at the moment only UUIDs are put there.

yep I am, but thanks for the information, I'll try to find a different workaround

no