How to modify attachment points?

How to modify attachment points?

I downloaded a model, and I want to attach a helmet to it's head. But it looks kinda off, the helmet is placed in front of the head, right in the face.
How do I modify the attachment points for the model, to make the attachment fit better?
I suppose I convert the model to .mdl and then use some tool, right?

Thank you.

Attachment points are part of the model, they're certain nodes in it

You'd need to use a tool that can modify the placement of bones and so forth

I have GMax installed, can I use that?
Btw, is there a tutorial or something for this?