Trim whitespace (spaces, tabs, newlines)
For anyone using SQL Server 2017 or newer

you can use the TRIM built-in function. For example:

DECLARE @Test NVARCHAR(4000);
SET @Test = N'  
    ' + NCHAR(0x09) + N'  ' + NCHAR(0x09) + N' this 
 ' + NCHAR(0x09) + NCHAR(0x09) + N'  content' + NCHAR(0x09) + NCHAR(0x09) + N'  
' + NCHAR(0x09) + N' ' + NCHAR(0x09) + NCHAR(0x09) + N'     ';

SELECT N'~'
        + TRIM(NCHAR(0x09) + NCHAR(0x20) + NCHAR(0x0D) + NCHAR(0x0A) FROM @Test)
        + N'~';


Please note that the default behavior of TRIM is to remove only spaces, so in order to also remove the tabs and newlines (CR + LFs), you need to specify the characters FROM clause.

Also, I used NCHAR(0x09) for the tab characters in the @Test variable so that the example code can be copied-and-pasted and retain the correct characters. Otherwise, tabs get converted into spaces when this page is rendered.

For anyone using SQL Server 2016 or older

You can create a function, either as a SQLCLR Scalar UDF or a T-SQL Inline TVF (iTVF). The T-SQL Inline TVF would be as follows:

CREATE
--ALTER
FUNCTION dbo.TrimChars(@OriginalString NVARCHAR(4000), @CharsToTrim NVARCHAR(50))
RETURNS TABLE
WITH SCHEMABINDING
AS RETURN
WITH cte AS
(
  SELECT PATINDEX(N'%[^' + @CharsToTrim + N']%', @OriginalString) AS [FirstChar],
         PATINDEX(N'%[^' + @CharsToTrim + N']%', REVERSE(@OriginalString)) AS [LastChar],
        LEN(@OriginalString + N'~') - 1 AS [ActualLength]
)
SELECT cte.[ActualLength],
       [FirstChar],
       ((cte.[ActualLength] - [LastChar]) + 1) AS [LastChar],
       SUBSTRING(@OriginalString, [FirstChar],
                 ((cte.[ActualLength] - [LastChar]) - [FirstChar] + 2)) AS [FixedString]
FROM   cte;
GO


And running it as follows:

DECLARE @Test NVARCHAR(4000);
SET @Test = N'  
    ' + NCHAR(0x09) + N'  ' + NCHAR(0x09) + N' this 
 ' + NCHAR(0x09) + NCHAR(0x09) + N'  content' + NCHAR(0x09) + NCHAR(0x09) + N'  
' + NCHAR(0x09) + N' ' + NCHAR(0x09) + NCHAR(0x09) + N'     ';

SELECT N'~' + tc.[FixedString] + N'~' AS [proof]
FROM   dbo.TrimChars(@Test, NCHAR(0x09) + NCHAR(0x20) + NCHAR(0x0D) + NCHAR(0x0A)) tc;


Returns:

proof
----
~this 
              content~


And you can use that in an UPDATE using CROSS APPLY:

UPDATE tbl
SET    tbl.[Column] = itvf.[FixedString]
FROM   SchemaName.TableName tbl
CROSS APPLY  dbo.TrimChars(tbl.[Column],
                           NCHAR(0x09) + NCHAR(0x20) + NCHAR(0x0D) + NCHAR(0x0A)) itvf


As mentioned at the beginning, this is also really easy via SQLCLR since .NET includes a Trim() method that does exactly the operation you are wanting. You can either code your own to call SqlString.Value.Trim(), or you can just install the Free version of the SQL# library (which I created, but this function is in the Free version) and use either String_Trim (which does just white space) or String_TrimChars where you pass in the characters to trim from both sides (just like the iTVF shown above).

DECLARE @Test NVARCHAR(4000);
SET @Test = N'  
    ' + NCHAR(0x09) + N'  ' + NCHAR(0x09) + N' this 
 ' + NCHAR(0x09) + NCHAR(0x09) + N'  content' + NCHAR(0x09) + NCHAR(0x09) + N'  
' + NCHAR(0x09) + N' ' + NCHAR(0x09) + NCHAR(0x09) + N'     ';

SELECT N'~' + SQL#.String_Trim(@Test) + N'~' AS [proof];


And it returns the exact same string as shown above in the iTVF example output. But being a scalar UDF, you would use it as follows in an UPDATE:

UPDATE tbl
SET    tbl.[Column] = SQL#.String_Trim(itvf.[Column])
FROM   SchemaName.TableName tbl


Either one of the above should be efficient for using across millions of rows. Inline TVFs are optimizible unlike Multi-statement TVFs and T-SQL scalar UDFs. And, SQLCLR Scalar UDFs have the potential to be used in parallel plans, as long as they are marked as IsDeterministic=true and do not set either type of DataAccess to Read (the default for both User and System data access is None), and both of those conditions are true for both of the SQLCLR functions noted above.