How to do update in AWS Dynamo DB using NodeJS

I have written this function to do update in dynamo table

const updateTask = async (req, res) => {
  try {
    const { existingTaskText,updatedTaskText } = req.body;
    console.log(existingTaskText,updatedTaskText );
    UPDATE({
      TableName: "todos",
      Key:{ task: existingTaskText},
      UpdateExpression:"set task = :task",
      ExpressionAttributeValues: {":task": updatedTaskText},
    });
    res.status(200).json({ data: "this is controller" });
  } catch (error) {
    res.status(400).json({ message: error.message });
  }
};

this is calling UPDATE

const UPDATE = async (payload) => {
  try {
    console.log(payload);
    const updateDoc = await dbClient
      .update({
        TableName: payload.TableName,
        Key: payload.Key,
        UpdateExpression: payload.UpdateExpression,
        ExpressionAttributeNames:payload.ExpressionAttributeNames,
        ReturnValues: "UPDATED_NEW",
      })
      .promise();
    console.log(updateDoc);
  } catch (error) {
    console.log(error);
  }
};

When I am testing this in postman, I am getting this error

ValidationException: Invalid UpdateExpression: An expression attribute value used in expression is not defined; attribute value: :task

this is payload log getting passed

{
  TableName: 'todos',
  Key: { task: 'see its  done' },
  UpdateExpression: 'set task = :task',
  ExpressionAttributeValues: { ':task': 'edited' }
}