Ordinarily, once a player posts her / his instructions for a round, that round is locked (so it cannot be edited) for review by the campaign's GM.

However, a player might accidentally post her or his round, when they only meant to save it.  There can also be instances where a player will belatedly realize that they have neglected to cover a matter in a round post.

There is a control for a player to ask the campaign's GM to unlock the round for editing.

This control can be accessed by viewing the latest round (via the links at the top right of the Game page), scrolling to the bottom of the screen and clicking Request GM to Edit link.

Whether to approve such a request is absolutely at the GM's discretion.  The GM is unlikely to approve a request if he / she has already  commenced work on the next round.