How to put methods onto the objects in Redux state?

In Redux, you don’t really have custom models. Your state should be plain objects (or Immutable records). They are not expected to have any custom methods.

Instead of putting methods onto the models (e.g. TodoItem.rename) you are expected to write reducers that handle actions. That’s the whole point of Redux.

// Manages single todo item
function todoItem(state, action) {
  switch (action.type) {
    case 'ADD':
      return { name: action.name, complete: false };

    case 'RENAME':
      return { ...state, name: action.name };

    case 'TOGGLE_COMPLETE':
      return { ...state, complete: !state.complete };

    default:
      return state;
  }
}

// Manages a list of todo items
function todoItems(state = [], action) {
  switch (action.type) {
    case 'ADD':
      return [...state, todoItem(undefined, action)];

    case 'REMOVE':
      return [
        ...state.slice(0, action.index),
        ...state.slice(action.index + 1)
      ];

    case 'RENAME':
    case 'TOGGLE_COMPLETE':
      return [
        ...state.slice(0, action.index),
        todoItem(state[action.index], action),
        ...state.slice(action.index + 1)
      ];
  }
}

If this still doesn’t make sense please read through the Redux basics tutorial because you seem to have a wrong idea about how Redux applications are structured.