Go into phpMyAdmin, go to the smf_members table, find the row for your account.
Edit that row in the following ways:
1. replace the passwd column with your new password
2. find the password_salt column and remove whatever value that column has for your row
If you then log in with the new password, it will tell you that 'password security has been upgraded' (i.e. it has taken the plain text password and converted it to the numbers/letters SMF uses) and you will have to log in a second time with the upgraded security version, but it'll let you in.