Once you've unziped OrientDB, start and stop the server one time (the default password for root is generated at first launching)
Next, open the file conf/orientdb-server-config.xml.
You should have a block like that:
<users>
<user name="root" password="QSDJQS7YDQSIODNL2NSJDKSdsjfkdshIQSUDBSQD" resources="*"/>
<user name="guest" password="guest" resources="connect,server.listDatabases,server.dblist"/>
</users>
The default password is in the password field (it's a plain text password).
You can simply replace it by the password of your choice, and restart the server.
Eg:
<users>
<user name="root" password="root" resources="*"/>
<user name="guest" password="guest" resources="connect,server.listDatabases,server.dblist"/>
</users>