Which HTTP method is used to replace a single letter in the login of a personal account and why?

To replace a single letter in a personal account login, it is better to use the HTTP PATCH method.

PATCH is intended for partial updates of a resource, unlike PUT, which usually replaces the entire resource. In the case of changing a single letter in the login, this is a partial change, so PATCH is more semantically correct.

Example request:

PATCH /users/123
Content-Type: application/json-patch+json

[
  { "op": "replace", "path": "/login/2", "value": "a" }
]

Here, we replace the character at index 2 in the login field.