How can I change value from a node when having a server instance (node-opcua)

alternatively you can use the setValueFromSource method on the variable. This will bypass all Read/Write access checking that takes place in writeValue. It is also synchronous

nodeToChange.setValueFromSource({ dataType: "Double", value: 3.14});

setValueFromSource can take an optional statusCode

nodeToChange.setValueFromSource(
     { dataType: "Double", value: 3.14}, 
     opcua.StatusCodes.BadWaitingForInitialData
);

if not specified StatusCodes.Good is assumed.

and a optional source timestamp

nodeToChange.setValueFromSource(
    { dataType: "Double", value: 3.14},
     opcua.StatusCodes.Good, new Date());