Can I exchange iPhone 5 16GB to 32GB? And how much it cost me? I am in UK

If you are still in the return window, then it can be returned and there will likely be a restocking fee. You will pay the difference between the 16 and 32GB prices.

If you are outside the return window (typically 14 days), the only option is to sell the current device and buy the larger capacity device you want.

I recall being told that the return window is 30 days for iPhones.

But still ... the OP should not waste time and bet on my estimate.