Write an LDIF file that contains all the Modifications.
You need to know the DNs of all the entries for which you want to reset the password.
Then for each DN create a record (single line separation between the records)
dn: cn=John,dc=example,dc=com
changetype: modify
replace: userPassword
userPassword: changeme!
-
dn: <another DN...>
...
Regards,
Ludovic.