Open SharePoint Designer 2013 and click Open Site.
In the Open Site dialog box, enter the address of your site and click Open.
In the Navigation pane, under Site Objects, click the Workflows option.
Select a workflow on your site to edit.
On the ribbon, on the Workflows tab, click the Edit Workflow button in the Edit group.
If you are first connecting to your site by using SharePoint Designer, you might be prompted to enter your credentials immediately after you connect to the site.