I have a hybrid bike which the frame is too big, is there any modifications I can make to make it more comfortable? I bought a hybrid bike from halfords around 6 months ago, however i have only recently had chance to ride it. I have found that the frame is too big, they wont exchange or give me a refund. I was wondering if there was any modifications i could make for it to feel more comfortable?
You have a couple of options, lowering the seat and changing the stem, or some combination of both. One other option would be to find someone that has a similar quality bicycle that is slightly small for them and trade! Local co-ops or bike clubs would be a place to start for this. 

As for changing the components you may consider an adjustable stem so that you can do the fine tuning your self, rather than buying a 70mm reach stem and realizing it didn't really help. 

You will have to decide what needs to change based on what part of your body is uncomfortable. If you feel stretched out like your reaching to far forward a shorter stem will bring your handlebars in a little. 

If you legs are being over extended drop the seat post a little and see how it feels. 

One other option would be to show up at your local bike shop with a 12 pack and ask for advice. They will charge for a proper fit, but with beer they should be fine with giving some advice and options free of charge. If they are not, find a new shop.

This site has some good information to explain it to beginners.

As does REI