There are couple ways to find the WOD you would like to edit. Lets say you want to edit your one of your recent posts, select your profile picture and from your most recent posts, select the workout you would like to edit. From the results page, select the blue edit icon, from the edit page edit your WOD and select save result.
Another way for you to edit your workout would be to head to "Todays WOD's", select the calendar and then find the workout from the exact date. Looking for a workout you did a few weeks ago or even a month or two ago will be easier than above (scrolling through your previous post). Once you selected your date, you can follow the same steps above by selecting the workout, selecting edit and saving your edited post.
Every result you log is editable. First find the result. The best places to look will be your Whiteboard calendar or your profile page. Once found, click on the result you originally logged. From the workout session page, click the "edit" button.
You'll be able to update the movements, the rep scheme, the weight, etc. It's the same as if you were posting the workout for the first time.