That's the best reason to change a signature! Congrats on the upcoming cruise.
Here are the steps:
On blue bar above the search box, all the way to the left, click User CP.
In the list on the left-hand side, click Edit Signature. It's under the subhead Settings & Options.
A box will come up that contains your current signature. Make the changes you want and then -- lots of people forget this next step -- Save Signature. (You can also preview it, but then make sure to save it once you're done.)