Editing a quote is dependant on what state the quote is at.  You can only edit quotes that are in the draft stage.  If the quote you need to edit has been issued, you will need to revise the quote before you can edit it.

In the quote dashboard's draft tab, click the quote number of the quote you would like to edit.

From the quote's detail page, click on Edit Quote from the actions buttons on the bottom of the page.

After editing the quote details, click Save or Save and Issue as required.