Resitting a year in A levels - need info on modular question

Hello. I am currently in my third year of college, resitting my A2 exams. I am on the Edexcel specification.

My first year I took -
C1 - A
C2 - B
M1 - U

Second year I took
C3 - D
C4 - E
S1 - A
M1 (Resit) - U


As you can see I got a U in Mechanics twice. When resitting this year, me and my teacher discussed this, and long story short I was entered for D1 instead of M1 instead, as we believe I have a better chance at getting a better grade.

I am wondering how this works when I get my grades this year. Assuming I now get an A in C3, C4, and D1, what happens to my overall grades? I was told they will take my best 6, but I am not sure how this works. For my AS will they replace my M1 with D1? Or with S1, and my D1 will count towards my A2 grade? As you can see I am very confused. Some help would be great.

Thanks in advance.