Its not a serious problem.
Note: Always make a backup before doing anything.
Open save/accounts.txt and change it there, it should be at the top. Make sure not to delete the tabs, make a backup copy first if nessesary. After that, delete the other s2/s3/s4/etc accounts.
Edit the configuration files char_athena.conf and map_athena.conf. Find the variables userid and passwd and change them to the values you placed on accounts.txt. After that, reboot your server and see if it works, if it doesn't restore the backups you made.