I've written a story in Ariel 14pt and it needs to be submitted in Times New Roman 12pt how do I convert?

Open the doent, use CtrlA to highlight the lot then up to the toolbar area, click on the downward arrow to the side of the font box which currently shows Arial, type TIM to bring Times New Roman to top of the list and click on it. Then click on downward arrow to side of font size box, and click on 12. Your text should now conform. I presume you have set it in double line spacing as is probably required also.