How to update the value stored in Dictionary in C#?

Just point to the dictionary at given key and assign a new value:

myDictionary[myKey] = myNewValue;

It's possible by accessing the key as index

for example:

Dictionary<string, int> dictionary = new Dictionary<string, int>();
dictionary["test"] = 1;
dictionary["test"] += 1;
Console.WriteLine (dictionary["test"]); // will print 2