Note: Your event needs to be in the status of Awaiting Responses. This means you cannot start the Scoring phase and then open the event back up.
So, to push the due date out in the future, click on the Details tab at the top of your RFP Dashboard.
When you click in the Response Deadline field, you'll be presented with a datepicker that you can use to move the deadline. Lastly, click on the orange Update RFP button at the bottom of the page to save.
Once that's done, all Points of Contact and Invited Vendors who have accepted your RFP will be notified via email that the date has been changed.
You cannot shorten a timeline, but you can extend it. Your respondents will be notified via email to the Points of Contact.