You can renew active medications for a patient within the Medications widget of their patient summary page.
Start on the summary page of the patient you wish to renew active medications for and navigate to the Medications widget.
Medications widget (click to enlarge)
Step 1: Within the Medications widget, under Active Medications, check the checkbox next to the medication you wish to renew. Then click the renew from medication icon that appears.
(click to enlarge)
Step 2: Alternatively, you can simply click the renew icon 
under the Actions column of the desired medication.
Medications widget (click to enlarge)