How hard is it to replace a car starter?

Replacing the starter itself is really easy... Just un-bolt it and remove the electrical connections and then do the reverse with the new one.

However it can be very awkward to get at all of the bolts and connections. It will take longer but it might be best to remove all obstructions that are in the way so you have better access to it, rather than struggling which may result in damage to yourself or components.