You should have received an email from AnyWhere Software containing a link to be able to download Version 6.80. Then there has been discussion about whether to just upgrade straight over 6.50 or to un-install 6.50 and install 6.80 fresh. I had no problem with a direct upgrade but the general consensus is to un-install first.
Once downloaded, remeber the importance of back-ups. V6.80 uses a different format for the .sbp file and whilst opening a 6.50 .sbp with 6.80 will be seamless and automatically converted - you won't be able to open a 6.80 .sbp with 6.50.
I'm sure Erel will be able to email you another link to the software download if you've inadvertantly deleted the first.