I'm afraid you can't change it yourself, unless you have all the necessary specialised equipment.
For a compressor or refrigerant fault, you'll need to get a refrigeration engineer in to sort it out.
Click here to find local engineers
Fill in your details and see if anyone comes up.
Alternatively,you may want to consider a repair through Domestic and General.
They will repair and then insure the product for a year.
www.domgen.com will get you details and phone number.
Check it out, see what you find and get back to me if you need more help
Hope this helps