Resetting a user's invidious password needs you to edit the database.
Firstly, generate a bcrypt-encrypted hash for the new password you want to set for the user.
This can be done with the bcrypt python module, though there are other ways of doing the same.
python3 -c 'import bcrypt; print(bcrypt.hashpw(b"<INSERT PASSWORD HERE>", bcrypt.gensalt(rounds=10)).decode("ascii"))'
To do so, first attach to the database:
# su - postgres
$ psql
postgres=# \c invidious
Now, run the following SQL query:
UPDATE users SET password = 'HASH' WHERE email = 'USERNAME';
After that, the password should be reset.