update any cell of field that equal 8 charachter with new string

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SQL

server

i have a table with below struct :

1-customer_id int
2-tel_number nvarchar(20)
3-tel_type tinyint

in tel_number field exists numbers like 44431223
now i want add string "76" to cells that has 8 character length (like 44331223)

example :
before execute query tel_number field contents is :

09167119004

44431223
44512112
44211451

09129874663

after execute query tel_number field contents is :

09167119004

7644431223
7644512112
7644211451

09129874663

How Do i Write SQL Query that do this operation ?

thanks