my_str = "Jane Will Rob Harry Fanch Dave Nancy"

changed_str = ''

for char in range(0, len(my_str)):
    if(my_str[char] == 'a'):
        changed_str += '$'
    else:
        changed_str += my_str[char]
print("原始字符串：")
print(my_str)
print("修改后的字符串：")
print(changed_str)

原始字符串：
Jane Will Rob Harry Fanch Dave Nancy
修改后的字符串：
Jne Will Rob Hrry Fnch Dve N$ncy