If you're following a workout plan, but you want to log a workout that is not your current plan, simply log a custom workout.
When you start the logging process, choose the "Custom Workout" option.
Then use the "Add Exercise" search box on the left side of the workout logger to add the exercises that you completed during your workout to your custom workout plan. Adjust the sets, reps, and weights as needed.
Finally, click "Log Workout," and your custom workout will be logged.