converting Epoch timestamp to sql server(human readable format)

I have 3 different columns with the same format. How can I change the values in those columns.

To update 3 columns in a table, you can pair DATEADD seconds to the epoch (1 Jan 1970) with the column name, i.e.

update tbl set
    datetimecol1 = dateadd(s, epochcol1, '19700101'),
    datetimecol2 = dateadd(s, epochcol2, '19700101'),
    datetimecol3 = dateadd(s, epochcol3, '19700101')

You can't update in place since a bigint column cannot also be a datetime column. You have to update them into 3 other columns.

Use the DATEADD function:

SELECT DATEADD(ss, 1291388960, '19700101')

...specifying a date of January 1st, 1970. In this example, it was provided in the YYYYMMDD format.

DATEADD will return a DATETIME data type, so if you have a table & column established -- you can use the function to INSERT/UPDATE depending on your needs. Provide details, and I'll clarify. Once you have a DATETIME to work with, you can use CAST or CONVERT to format the date in TSQL.