They are a pain to change - especially if your GT has aircon. I also gave up the first time I tried and came back to it a few days later. Although it doesn't appear possible, you need to remove the plastic covers from the back of the lamp units. It's like one of those kiddie puzzles, but it is possible to slide them out completely. Having done that, look for the long black plastic bulb holders with a paddle on and pull them out. Don't touch the glass on the new bulbs and turn the bulbs on before you try to refit them, so you can see where they are.
Don't fit sidelight bulbs from a well known high street store that offer to fit bulbs for you, because the quality is poor and they will blow again very quickly. After changing mine three times in 6 months I gave up and fitted 'error free' LED bulbs from Ebay. These seem to be working well at the moment, but it's early days.