coolant question

while driving my wifes car today(96 fiat cinquecento) temp guage went thru the roof all of a sudden, pulled in straightaway and opened bonnet, steam etc, like a cow lookin over a wall so used breakdown
assist(covered by insurance), only waitin for bout 30 mins, guy took quick look and showed me that the hose had come free from rad, told me this caused coolant to leak and rad not to kick in? reconnected hose, filled up with water, pressed a few other hoses then started car, after
few mins told me everything was ok and all i needed to do was add some coolant when i got a chance? heres my question, do i need to empty coolant reservoir and then add new coolant or can i just top up with coolant? was so happy to get goin so quickly didnt think to ask the very nice man that came out to me!!!!
tia
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If you lost a lot of coolant, and it was then filled up with water, then you need to drain and refill weith anti freeze in time for winter.
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