You can roll over your open trade by clicking on the last column "Instr R/O" Iin your "Open Trades" window and selecting "Yes" or "No" to have your position rolled over, please see explanation below:

Roll-over explained:

NB: Unless you select instrument roll-over on the trading platform or
contact the trading desk, your open position will be closed out at the
published expiry price / - GT Shading.

Rollover Process Explained:
Your open Sep08 positions will be closed out at the middle of the GT spread,
hence realising a profit or loss on the position.
Your Dec08 position will then be entered at the GT Bid or Offer (depending on
whether you had a Long (Buy) or Short (Sell) position).

Note: Should this Rollover result in you realising a loss on that position you will
be required to replace the lost margin = to the loss incurred in order to maintain
the original Risk. Should you not have sufficient funds available to facilitate this
process your Risk will be reduced to a level that you are able to fund.

Stop Loss: The stop loss in place for the trade in the expiring contract will be
defaulted to an auto stop level (this may result in an additional margin
Take Profit: Take profit levels will need to be re-entered when the position has
been rolled into the new contract.