Unless you can change it in "Edit" the quickest way is probably to repost it, and using the hourly job as the template, changing hourly to fixed rate.
Close it, then post job, reuse previous job, pick from the drop down menu, then change to fixed rate under "How would I like to pay"
Also, if anyone has already applied to your job, it might be nice to send them an invite so they haven't wasted their connects. You could just leave them a message stating that the job change is why you are sending them the invite.
Thanks but if I do that then I lose the list of people that applied so far. I would not be able to save the 2 freelancers who I'm waiting to hear from but I'd also like more selection.