Q1:
greeting = 'Hello, world!'
new_greeting = greeting[:7] + 'm' + greeting[8:] 
print new_greeting 

Q2:
greeting = 'Hello, world!'
new_greeting=''
for base in greeting:
   if(base != 'w'):
     new_greeting= new_greeting + base
   else: 
     new_greeting= new_greeting + 'M'

print new_greeting