To update your iDevice to the last available version of iOS go to:
General - if there is a newer version available, a notification is displayed next to the label
The details of the version are displayed, tap "Install now"
Read and accept the Terms and Conditions
The installation starts
Your iDevice will reboot
All done, enjoy the new version
It's also possible to update iOS to the latest version by plugging your iDevice into your computer. Launch iTunes and select your device. You can then check for the latest update and install it.
During the update, if prompted you should reset the carrier profile - this is the default answer. This will make sure that you are using the most up-to-date Internet and MMS settings.
This will replace your current profile with giffgaff's Internet settings, including the parameters which can't be modified manually (e.g. proxy settings). MMS settings must be modified manually, see below.