Actually if i add the clean code the script wont work and also i did'nt modified the script i just added the filenames
You misunderstood the instructions, you are not supposed to add your file names into the code.
if Time.now.mon==2 or Time.now.mon==3 or Time.now.mon==4
When you call that function, the name you give to the function is automatically placed behind spring_.
For example,call the function with name 'hello.png' and you'd get Graphics/Tilesets/Spring_hello.png
You're not supposed to manually go in and add your file names where it says name, that happens automatically when the function is called.