Honda Civic Service Manual: Engine Cooling

Engine

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Coolant Replacement (R18A9)

1. Radiator Cap 1. Wait until the engine is cool, then carefully remove the radiator cap. ...

See also:

Honda Civic Owners Manual. Adding the Coolant
Make sure the engine and radiator are cool.  Turn the reserve tank cap 1/8 of a turn counterclockwise and relieve any pressure in the cooling system.  Push down and turn the reserve tank cap counterclockwise to remove it.  Pour coolant into the reserve tank until ...