New download, cant change the realmlist

Hello guys, I wanted to play on this server and im new here. I downloaded everything and wanted to change the realmlist from "set realmlist logon.molten-wow.com" to "set realmlist logon.warmane.com".
But whenever I try to change this, I cant safe the new realmlist there, I gotta need to make a new file for this. But even with the new file realmlist I cant play on the WOTLK Realm.
So is there any way to avoid making an new file for the new realmlist? The old realmlist is still in the file, thats probably the reason I cant play there.

Delete old file, create new WTF file (not txt).