You can take a bus from Marlow to Amersham via High Wycombe, Bus Station (Shopping Centre) in around 52 min.
How to get from Marlow to Amersham
From Marlow, Spittal Street (Liston Road) bus to High Wycombe, Bus Station (Shopping Centre) then bus to Old Amersham, Whielden Street (Whielden Green). 52 min £5. From Marlow taxi to Amersham. 19 min £68. From Marlow drive to Amersham. 19 min £2.