Need a little help in automating syncronization

Hi i am new to winscp command line
I am trying to automate a process, let me explain in detail

1) i will connect to sftp and download each and every excel file in specific location to my local drive ( successful in doing this)
2) then i will add extra information to the downloaded file in my local drive
3) then now i need to replace the appended file with the original file in sftp location ( Not able to figure out how)

guide me what commands are to be used to replace the original file with the appended file using
Synchronize command

how to compare the timestamp of the file available on sftp with that of my local drive file and then the latest file should be replace the old file.

In my case i will always append the file on my local and obviously my local file will be latest file... before the synchronize starts


Thanks and Regards
Trinath