Help

I've never made a website but recently a business i work for asked me to make a website and ive completed the layout on dreamweaver and they already have a website their paying for they just want the new information ive made put on it but im confused on how to use smart ftp.

when i open smart ftp they have a login name and pass and i see the current files on the website do i delete those and upload the files ive made?

and someone told me that i shouldnt delete all of the files beacuse some of the files are already their at the start. So can someone tell me what those files are?

they bought for me studio 8 so if that can in any way tie together in helping me please let me know.

im all together confused any help would be very appreciated