Unless it has long life coolant (red?) it should be changed every two years. Drain, flush with clean water, refill with new coolant (should use rain water or distilled) at right mix for your part of the world. Ensure the old coolant and water used for flushing does not enter any open water course or soakaway. It must go though a water treatment plant.
I suspect that the injector service is just a dose of Forte injector cleaner or similar (Wynns, STP) in the tank. They will bad mouth anything they don't supply as being likely to degrade the injector seals and cause leaks.
Use a turkey baster. Suck old stuff out, re-fill, run a few days and then do it again. You don't get a 100% replacement of fluid but it's good enough and better than taking the oulet hose off, runing it dry and then having to bleed the air out after refilling.
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