There is no provided way way to alter the value of a key as MSDN states:
Dictionary<TKey,TValue> Class (System.Collections.Generic) | Microsoft Docs[
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Quote:
As long as an object is used as a key in the Dictionary<tkey,tvalue>, it must not change in any way that affects its hash value. Every key in a Dictionary<tkey,tvalue> must be unique according to the dictionary's equality comparer. A key cannot be null, but a value can be, if its type TValue is a reference type.
Which means that for value types such as int you cannot modify the key as that would change the hashcode (and value types are derived from object).
For reference types it's less cut and dried as the GetHashCode method can be overridden so any changes need to be executed with extreme caution or "unpredictable results may occur".
You would have to delete the original and add a new one.