Change value for Field B when value for field A changes

Hello Team,

I would like a field (let's name it Field B) to change its value once a field on the same form (Field A) is changed.

Field A: Region (Values: USA,Europe,Asia,Africa)

Field B: Section (Values: Sect1,Sect2,Sect3,None)

So, in detailed, when Region field is changed to Africa, the Section field should automatically change to None and becomes uneditable.