On the Auction Transaction List, find the transaction you wish to copy, click in the radio button on the left-hand side and then on the word Copy at the top.

The copied auction transaction is added to Auction Transaction List in the state In Progress, click on the green System Ref of the new transaction to go into it and make any necessary changes – which will include setting a new End Date and Time for the Auction.  When everything is correct, click Update Auction at the bottom and continue to process the transaction as normal.