That's what I did. Siphon some of the coolant off via the overflow bottle (OB) using a small electric drill driven pump, to clean out the contents of the OB.
Also remove the overflow line from the OB and use that to pour in the WW. In this manner, the WW is added into the engine block directly. Of course, the fluid is displaced and you see it rise up in the OB.
If you know you have 50/50, then this should be ok since there's about one gallon in my OB. To get more out, you could open the radiator drain plug to collect say 1-1/2 gallons of coolant. Then add the WW and water via the overflow line.
For my radiator, I used 1-1/2 bottles of WW.
Regards . . . . JimF
'94 S500 Cpe
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