Here's the absolute easiest way to upgrade clients with a new version NOW:
1) Assign the package to the group that you want to update (Look in the Clients section, under the Install Packages tab). Do not set a schedule and make certain that you are assigning the correct version (64-bit for X64 clients and 32-bit for X86 clients).
2) Go to the Admin section>Servers>Site, and click on Download LiveUpdate Content. This will kick off a LiveUpdate check on the SEPM server, which will pick up on the new client install package. Your clients will start pulling and installing on their next check-in.
Two quick caveats:
1) I've always found the scheduling feature to be a bit flakey, so avoid using it personally. Never quite seems to do what I was expecting.
2) If you find that a client here and there doesn't update with the rest, to my experience, chances are that the client(s) already has an installer change scheduled for the next reboot. This will cause the SEP installer to quit. In these cases, I just remote reboot the client and then the new package will install just fine. Changes waiting on a reboot are found in the registry at HKLM_SYSTEM\CurrentControlSet\Control\Session Manager. Look for a key starting with Pending.