Bluetooth 2.1 to 4.0

First there is no Apple upgrade for your system.

Second, depending on your model you could have the Bluetooth board buried in the clutch cover (Glossy display model) which would require almost completely disassembly of your system to get to it.

As for options, you could get an external Bluetooth 4.0 USB transceiver unit if you have a device that requires it.

If on the other hand you are thinking about Yosemite Handoff services you'll need to wait to see what happens with 3rd party options. As what is known right now, external options are not possible. Some of the newer MacBook Pro systems which share the same hardware design which have Bluetooth 4 LE interfaces can be upgraded. But, to gain Handoff you'll need to alter the OS kext files to make it work.