When you come across the string question the interviewer may always 
increases the level 1 by 1 to test your knowledge.

Replace first occurrence of letter with another  is one of the favorite question in interview room.


Let try to break it 
  • find is a method which will give us the first occurrence of letter index 
  • As string is not mutable [ cannot make any changes] , we may need to convert it to list 
  • After converting to list , can easily perform assignment & reconvert back to string via join  

Code 

# Replace 'i' with 'k'

strname='python is simple'
replaceLetter = 'i' 

replaceLetterIndx=strname.find(replaceLetter)

lst = list(strname)
lst[replaceLetterIndx] = 'k'

print(''.join(lst))

Note : You cannot make use of replace method as it is going to replace all letters wherever it is coming in a string.