If it's this model
https://www.argos.co.uk/product/9428463
it should have the ITV Hub app on it, which will hopefully automatically update to become ITVX without you having to do anything. However some smart TV sets aren't compatible with the ITVX app. If ITV Hub doesn't auto-update to ITVX, yours is one of them.
If it's this TV set
https://www.argos.co.uk/product/9359460
then it's not 'smart', meaning that (on its own) it can't be used to access any streaming services at all.
If you've got the first set above and you find that ITV Hub doesn't become ITVX without you taking any action, or if you've bought the second set above, you'll need to buy an external device (such as one from Roku or an Amazon Fire TV Stick) in order to view ITVX.