I demand an exchange!

After owning an iMac for 7 years, without trouble, I bought another one. Bought a demo model as I was told it is basically brand new and they didn't have stock of the model at the time, so I decided to go with it. It has now been only a few months and I have had to take it back 3 times to have it repaired! Each time I get charged for software installation, but it doesn't sort out the problem. I don't believe the technical department at Incredible Connection in Worcester has the expertise to repair the computer. It is under guarantee and it should have been sent away to Apple to have the problem fixed. All the money I have spent on this computer surely makes up for the little bit of 'demo' discount they gave me, and it is still not in proper working condition. I want a new computer. There is no point paying for something that I can't use. And one last thing. The screen is chipped. I am 84 years old and I didn't see it on the day of purchase. I did take it back a week later and they simply said it wasn't there when I bought it. At my age, I don't see why I should keep quiet and accept this. As a loyal client, I feel cheated. Now, what are you going to do?