how can i change my roof?

hi can anyone help,i bought a second hand roof in red as my old roof was blue and window was cracked so thought i would go the hole hog and change the hole roof and colour. my car has been with macanic for two wks now, and today he said that the red roof doesnt fit and one of the pipes keeps flying off, he said it doesnt sit properly at the back, so he has now put my blue roof back on, this has realy **** me of, does anyone now how to change a roof over