I need to make changes to a Workflow, but I currently don't have access to it. Which permissions are required?

The user needs to be given "Designer" rights by the account which created the Workflow. Please refer to the following articles to learn more about DocuWare Workflow rights and how they are granted to users;

Workflow Rights Overview
KBA-36664  · DocuWare Support Portal

How to grant Controller Rights
How to grant Designer Rights 

Once given the Designer rights, the user will then be able to access and make changes to the workflow.
NOTE: Please ensure to select the Workflow prior to granting rights since they only apply on a workflow-by-workflow basis.

KBA is applicable for both Cloud and On-premise Organizations.