iOS7 is due to be available for download in the UK after 18:00 this evening. There are two ways in which you can easily update your devices;
Go into Settings>General>Software Update
When the update is ready it will give you the option to Download and Install it.
Update using iTunes
Plug your device into iTunes
Select your device and under the Summary tab you can choose to check for update
I recommend that you backup your device first before the update.