For users who have view only or comment only access to a doc, we've created a button to request access from the doc owner.
To get edit access, just hit Request edit access in the top right of the doc:
If you don't see the button, that's because the doc owner has disabled edit requests. If you're a doc owner and want to disable the ability to request access, toggle this setting in the Share menu off:
All editors of a doc should have access to update this setting. Note that the Request edit access button is never shown on the published view of a doc, no matter what that option is set to.
See Sharing your doc for more details on how sharing works in Coda.