How to update access table

[ANDRE726]

I have 4 columns of interest in my table
userid...number
groupcode...text(10)
startdate...datetime
enddate....datetime

records are organized by userid by groupcode
example

125 AH0055A 1/22/2010 08:00 AM 1/22/2010 08:15 AM
125 AH0055A 1/22/2010 08:00 AM 1/22/2010 08:17 AM
125 AH0055A 1/22/2010 08:00 AM 1/22/2010 08:19 AM
125 AH0055A 1/22/2010 08:00 AM 1/22/2010 08:22 AM


125 AH0055A 1/22/2010 08:00 AM 1/22/2010 08:25 AM
125 AH0055A 1/22/2010 08:00 AM 1/22/2010 08:29 AM

Each group has the same startdate throughout with different enddate. What I am trying to do is to change the next startdate/time to the previous record enddate/time. Results would look as follows

125 AH0055A 1/22/2010 08:00 AM 1/22/2010 08:15 AM
125 AH0055A 1/22/2010 08:15 AM 1/22/2010 08:17 AM
125 AH0055A 1/22/2010 08:17 AM 1/22/2010 08:19 AM
125 AH0055A 1/22/2010 08:19 AM 1/22/2010 08:22 AM
125 AH0055A 1/22/2010 08:22 AM 1/22/2010 08:25 AM
125 AH0055A 1/22/2010 08:25 AM 1/22/2010 08:29 AM

I can do this manually for 10 or 20 records but I have 25,000 records to update. Can anyone enlighten me as to how this can be done. Any help would be greatly appreciated.

[Rainlover]

The End Date is Redundant Data.

If there is a new Start Date then the previous should be ignored.

The End Date is in actual fact the next Start Date.