The error suggest that your password is incorrect (or that the SYSDBA user doesn't exist, but that is unlikely). You need to use the correct password. Likely you specified a password in the installer.
If you don't know the correct password, it is not possible to change it with Firebird 2.5 on Windows (unless you have an account with the RDB$ADMIN
role and know that password). You will need to replace the security2.fdb with one that has a known password, e.g. from the Firebird 2.5 zip download, where it is 'masterkey' (or actually, 'masterke'). However, be aware that doing this will make you lose all other users and passwords as well.
See also Forgot the username and password of *fdb (firebird) database. Is there anyway I can crack this database?
As an aside, if you're new to Firebird, I would recommend starting with Firebird 4.0 instead. Firebird 2.5 is end-of-life.