Up to Q2A v1.7.4 this was the only way:
I ended up running the following update on the database:
UPDATE qa_users
SET passcheck = UNHEX(SHA1(CONCAT(LEFT(passsalt, 8), 'new_pass', RIGHT(passsalt, 8))))
WHERE userid = 1;
Replace 1 with the user's id and new_pass with the new password for the user.
As of Q2A v1.8 things have changed. This should be the approach to use:
1. Get the password from the PHP function password_hash (you could use a PHP online IDE to do so quickly):
echo password_hash('new_pass', 1);
That should return an awful string. For example, after hashing password P@5sWoRd you should get something like this $2y$10$xkK9WkpgacLrE4kakOZFmO/1SxLWq6BvJYSjAC1GAZnKShSQbjm.O
2. Once you have the hashed password run the following query:
UPDATE qa_users
SET passhash = 'hashed_password'
WHERE userid = 1;
Replace 1 with the user's id and hashed_password with the hashed password generated in the previous step.