Please note this will affect ALL the extended information links for your store.
Go to 'Settings | Site options | Extended Information', against ' Type of Extended Information Page' use the drop down to select 'Opens in The Same Window' as a layout.
Click on 'Apply' then 'OK' to preview.
This can however be overridden at the Product level on the 'Extended Information' tab, select an alternate layout using the drop-down and 'Apply'.
Go to 'Settings | Site options | Extended Information', against ' Type of Extended Information Page' use the drop down to select 'Opens in The Same Window' as a layout.
Click on 'Apply' then 'OK' to preview.
This can however be overridden at the Product level on the 'Extended Information' tab, select an alternate layout using the drop-down and 'Apply'.