Besides the hourly contract weekly payments or milestones payments, you can issue a bonus or an extra payment to your freelancer.
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 "Give bonus/reimbursement":
Clicking there, a window will open, where you can fill in all the information:
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.