If somebody will get into this trouble, so here’s the solution:
UPDATE dbo.Readers_new
--using STUFF to replace Contact_telephone's value on 499 with len = 3 starting from 2nd index
SET Contact_telephone = STUFF(Contact_telephone, 2, 3, 499)
--where Contact_telephone's value = 495 starting from 2nd index with len = 3
WHERE SUBSTRING(STR(Contact_telephone , 11), 2, 3) = 495
--select everything from Readers_new table
SELECT * FROM dbo.Readers_new