You will need the Full Write Workflows permission or be an admin on the associated workflow.
Navigate to the workflow and then the process you want to edit. Click on "Edit process details" in the "Actions" menu in the upper right area.
A pop up will appear where you can edit the name, description, and trigger for the process. When done, click on "Save" to finish.
Helpful Tip
See this article to learn about editing the tasks on a process.