Once you have activated a Flow and you want to modify it, Salesforce doesn't allow you to do it. There are two possibilities (mentioned in step 1 and 2):

1. Create a new Flow by copying the activated Flow.
2. Modify the activated Flow and save it as New Version or save the activated Flow as New Version and modify it. Once you are done with the modifications, activate the new version of the Flow.
3. You can have multiple versions of a Flow, but you can only activate one version at a time. The Flow Detail page allows you to activate or deactivate one version of a Flow. Click on the Activate link, available next to version 1 of the Flow, as shown in the following screenshot:
   
   

4. From the Flow Detail page, copy the unique URL; it will look like /flow/Create_Lead.