Maybe I'm wrong, but there is no official Ubuntu package of Samba 3.5 in repositories yet.
You have two options:
1. Wait for official package
2. Compile new samba Yourself
3. Use untrusted sources
If waiting or compiling isn't solution (time is money :) ) then you could use untrusted repository.
At first I will write big warning to not be responsible if something happen with Your system :) :
I DOESN'T TEST THIS REPOSITORY. USE IT AT YOUR OWN RISK.
Then check Your Ubuntu version.
This package is designed for Ubuntu 9.10
If everything is ok then prepare 3 easy steps:
1. Add repository:
sudo add-apt-repository ppa:polslinux/ppa
2. Update package list:
sudo apt-get update
3. Then upgrade samba:
sudo apt-get upgrade samba
Repository info: https://launchpad.net/~polslinux/+archive/ppa