Talking to obgyn about changing cs date.. advice

Can anyone give me advice on how to talk to my obgyn about changing the caesarian date. He has got me booked in for my 4th cs at 40 weeks and 1 day ( he will be doing the cs i think) with my last pregnancy my scr was very thin which makes me so anxious because I had her at 38 weeks not 40. He has got my due date 26th Jan... I dont even no where he got that date from because my dating ultrasound shows I'm due on the 20th and from my lmp I'm due on the 23rd and I spoke to my midwife friend who knows my ob and she said to talk to him and tell him to move the cs date forward because his cs is wrong but I'm scared he will take it the wrong way and get mad because he's already sent the paperwork off. How do I talk to him about it?