Here's how you do it (using .WRITE):
/*
.WRITE in the update clause like substring
*/
CREATE TABLE TempDemoTable (BigCol varchar(max))
Insert into TempDemoTable (BigCol)
Select 'Now is the time for all good men to come to the aid of their country';
Select * from TempDemoTable;
-- note: partial updates to large value data types using the .WRITE clause are minimally logged
Update TempDemoTable
Set BigCol .WRITE ('women', 29, 3);
Select * from TempDemoTable;
-- length is null
Update TempDemoTable
Set BigCol .WRITE ('women', 29, null);
Select * from TempDemoTable;
-- expression is null
Update TempDemoTable
Set BigCol .WRITE (null, 29, 3);
Select * from TempDemoTable;
-- offset is null
Update TempDemoTable
Set BigCol .WRITE (getdate(), null, null);
Select * from TempDemoTable;
DROP TABLE TempDemoTable