Countdown Timer

Navigate to CMS -> Static Blocks

Find block with ID: timer

In here you will see some short code that looks roughly like this;

<p>{{block type="zero1_smart_blocks/countDownBlock" name="countdownblock" count_down_to="13:00" message_before="Order In The Next" message_after="And Get Next Day Delivery"}}</p>

You only need to note the count_down_to="13:00" part.

Simply change the value to a different cut off if required, e.g.

count_down_to="14:00"

count_down_to="12:00"

You may need to clear the cache manually to see these changes.

Lead Time

If you need to change the lead time message to match, e.g. 

"NEXT WORKING DAY DELIVERY AVAILABLE IF ORDERED BY 2PM (MON-THURS)"

Navigate to Catalog -> Attributes -> Manage Attributes

Filter the list to find attribute code lead_time;

Click into this and select Manage Label / Options on the left hand side.

Under the English heading find the Next working day entry and amend the time here.

You may need to clear the cache manually to see these changes.