Question

how to convert methyl cyanide to acetone

Answer 1

$\huge\bold\red{Hello!!}$
Stepwise Conversion   (Reagents are in the brackets)

Methyl Cyanide => Ethanoic Acid    (H3O+)
Ethanoic Acid => Ethanol    (LiAlH​4)
Ethanol => Chloroethane     (SOCl2)
Chloroethane => Ethyl cyanide (KCN)
Ethyl cyanide => Propanoic Acid  (H3O+)​
Propanoic Acid => Propan-1-ol  (LiAlH​4)​
Propan-1-ol => Propene    (H2SO4,180oC)
​Propene => Propan-2-ol    ( Hg(OAc)2, THF, NaBH4, OH-)
​Propan-2-ol => Acetone  (PCC)

Hope you understand PCC - ​Pyridinium chlorochromate.