The "standard" build is listed first and is the default firmware that the pyboards are shipped with. Use this firmware if you are uncertain. The "double FP" builds use double-precision floating point instead of the standard single precision. The "threading" builds contain the _thread module and allow multithreading. The "network" builds have network drivers for CC3000 and WIZ820io included. All these different firmware are completely interchangeable and you can freely change from one to the other without losing the filesystem on your pyboard.
One you have downloaded the appropriate DFU file it can be flashed directly to your pyboard
using a DFU programmer. You can enter the DFU bootloader on the pyboard by executing
machine.bootloader() at the MicroPython REPL.