Updating Dictionary Values in C#

In C#, you update a value in a Dictionary by assigning to an existing key with the indexer — it overwrites the old value:

Dictionary<string, int> scores = new Dictionary<string, int>();
scores["Alice"] = 95;
scores["Bob"] = 87;

scores["Alice"] = 99;   // update: Alice is now 99
scores["Charlie"] = 72; // add new entry

scores[key] = value is used for both inserting and updating. To update only existing keys, check with ContainsKey() first:

if (scores.ContainsKey("Alice"))
{
    scores["Alice"] = 99;  // only updates if key exists
}