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", where you'll find the list of all your active contracts.
Click on the three little vertical dots on the right of the freelancer whose contract terms you want to change, and you'll find the option "Rate/Hours Change":
Clicking there, a window will open, where you can propose the new terms (you can change the hourly rate or the weekly number of hours the freelancer can register. Click "Suggest Changes" to complete the action.
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.