You can edit any previously logged result. It's simply a matter of finding the workout you want, and clicking on the "edit" button.
To find an old workout, you have three options. First, click into your profile to view a quick feed of your most recent 10 posts. This is an easy way to get to a recent workout quickly.
You can also click into the Activity Feeds icon and then again into the "Me" tab. The "Me" tab will show you ALL of your logged scores in reverse chronological order.
Lastly, you can find old results from the calendar via the Fitness tab on the home screen. Click into a previous day to find logged results on the various tracks you follow.
Once you find the workout you want for each of those three options, click on the linked score, or the "View Details" link. Either will take you to the Workout Session screen.
Click on the "pencil" icon in the top right corner of the Workout Session screen. This will take you back through the logging process for the workout so that you may edit (movements, notes, score, etc) as needed.
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.