I changed the phone's security(which is used to lock the phone to prevent others for using it) code. Is there any way I can reset it?

By changing it again.

If you've forgotten it, then reflashing the phone is needed.

How about restoring the phone to its default?

And how would you do that without the security code?

*#7370#, I saw this from other post, but don't know if this will erase everthing in the phone memory or not.

Even if you want to do a hard reset(*#7370#),you still need a security code.