You can change the terms of the job offer every time you need to.
To do that, log in to your account and go to the My Jobs - Active Jobs tab and click on the job whose terms you want to change:
On the next page, click on the three little vertical dots on the right of the freelancer, where you can find options regarding the job, including "Suggest rate change":
Clicking there, a window will open, where you can propose the new terms.
You also find all your jobs on your dashboard as soon as you sign in, where you can also access the options next to each freelancer you hired.
The freelancer will be notified about the new terms and has to accept them in order for them to be active.