How to vary output of DC power supply?

I have a DC Power Supply, Model 20R, 120 input, 13.5 DC output, 20 amp.  Is there a way to change the output Amps?  I want to use this power supply for nickel plating. I need the amps to be variable or fixed at more than the onr setting of 20 amps.  (example 2, 4, 6, 8, 10 amps )

This is a nice heavy duty PS made in USA.  A new DC PS is over $400 and they are cheaply made imports. I used to repair electronics for NASA years ago.  I most likely would have known how to do this back then, but the older I get the more I can't recall. (Old age is not fun!!) My point is, I have the experience to do what you tell me to do.

You need to add a series current regulator, but it will have to be substantially heatsinked.

I suggest you use an LM317 regulator, but beef it up like the circuit on page 22 of this link

Add more 317s until you get the limit you need. Using multiple devices will make it easier to source the bits

The "cheap imports" are nice variable switch mode PSUs, and will waste a lot less heat than a series reg. You'd make it work in current mode by using the external control terminals on them and adding a series pass resistor that you pick up the current with.