I have a existing exhaust fan in my bathroom that I would like to replace, can I change it out just from the bathroom or do I have to go into the attic?

Anthony,

The way you phrased your question makes it impossible to answer. I have no idea what the existing fan mounting looks like from here.

Nor can I see whether the fan is a self venting fan or vents to the outside of the roof (as it should) to prevent mold from growing on the underside of the roof.

Can you give us some help here, please?

I will say, some fans can be removed and maneuvered down from below, but it is tricky. It is by far much easier to do from the attic.
If you've never done it before don't even think about doing it from below.

Depends on how the fan is mounted to the joists. In some cases they are inside the joist and in others they are mounted to the bottom of the joist, in which case you would need to remove some drywall. Can you see how the fan is mounted?