How do I backup IPhone 4.2.1 prior to trying to sort out problem which won't allow Apps to be opened although bought and icons appearing os phone screen
The same way you backup a device running any other version of iOS.
Connect to a computer running iTunes.
Right click on the device and select backup.
This is covered in the article from which you posted your question.
Many thanks for the advice - it was the right-click bit that fooled me
Retrieving data ...