P6 is not a simple tool to use and I always recomend new users have some structured training. Also some of the functions do not operate as one would expect.

It is my opinion that:

1. Users should never use Update Progress as it will chanage actaul start date of inprogress activities to the Planned Dates without warning. To prevent this happening some people run a Global Change to set the Planned Dates to the Current Scheduled dates before running Update Progress.
2. Users should ignore the Budget Costs and Units and always use the Baseline values to record your original Budget Costs and Units. This is because the Budget Costs and Units will always change when you change the At Completion values before an activity has an Actual Start.

Regards

Paul E Harris
Director, Eastwood Harris Pty Ltd
www.eh.com.au

that is not how you update a schedule 

1. on the status tab if an activity has started but not finished you check the stated box then select the drop down window and select the date that it actually started on. next you put in the remaining duration in terms of work days not total day but the number of work days it will take you to complete the remainder of the work for that activity.

2. on the status tab if an activity has started AND fnished you check the stated box then select the drop down window and select the date that it actually started on. Then you check the box next to the finish date and from the drop down select the date that the ACTUAL date the activiity finished on.

3. Add any new activities and logic.

4. Change the data date to reflect the date that you have recorded progress up to and then re-scheduel the project.

5. Review for accuracy make sure what you are reporting matches the current conditions.

6. Fix any errors. 

7. Get buy-in 

8. Publish and distribute reports and schedules to interested parties.

9. Done

in the Activities Directory it is in the General tab

Check your settings. I think you have opted "Duration" for activity % complete type.