The easiest way is to download fp64lib via the builtin Library Manager of the Arduino IDE, just search for fp64lib. If you are not familiar with the Library Manager, see here for instructions.

You can also download the lastest version of the library here, that can be directly included into your Arduino IDE:

If you want to explore the source code, go and check the github repository:

fp64lib Source Code on github

Please use also the github page to report any issues/bugs.