how to change output impedance of op amp
We only tend to need 'impedance matching' when dealing with RF, and an LF351 is not an RF amplifier.

50 ohms is quite a low load impedance for an opamp to drive, so the main part of driving a 50 ohm load is to make sure it will source enough current.

With a +/- 15v supply, the 351 is specified to drive a minimum of 10v, and typically 12v, into a 2k load, so let's optimistic and use typicals and say 6mA. If we look at the short circuit current specification, it's a minimum of 10mA, and a max of 60mA, so until limited by device dissipation, and for any particular device, you may be able to supply a little bit to a lot more.

+/- 6mA into a 50 ohm load is a swing of only +/- 300mV. 

If you constrain your opamp gain such that it never drives more than +/- 300mV, you could connect it directly. Alternatively, put a resistor in series with the 50 ohm load, drive more voltage into it, and let the resistor and load divide it down to 300mV at the load.

If you do actually want to make the output 50 ohms impedance, say to drive a long transmission line without reflections, then you would configure the opamp to produce twice as much voltage as you want the load to see, and use a series 50 ohm resistor. However, for LF351 frequencies, it would have to be a verrrry long transmission line to make it worth the trouble.

Note that some opamps are not stable into a very low load impedance, I don't know about the 351 specifically, you'l  have to try it and see. If it's unstable driving 50 ohms directly, or even the 100 ohms of a series 50ohm and the load, then increase the series resistor until it's happy, up to 2k as the data sheet suggests.