How to set bool value in SQL

You need case statement with when and else if not any condition satisfied

Update [mydb].[dbo].[myTable]
SET isTrue = ( CASE WHEN Name = 'Jason' 
                   THEN 1 else 0 
               END)

Sql server does not expose a boolean data type which can be used in queries.
Instead, it has a bit data type where the possible values are 0 or 1.
So to answer your question, you should use 1 to indicate a true value, 0 to indicate a false value, or null to indicate an unknown value.

Update [mydb].[dbo].[myTable]
SET isTrue =
CASE WHEN Name = 'Jason' THEN 
    1
ELSE 
    0
END

The query you added will work fine, but it will you have to take care of "FALSE" part as well otherwise it will try to enter NULL in your column. Do you have any default value constrain on isTrue column?

Update [mydb].[dbo].[myTable]
SET isTrue =
(
   CASE WHEN Name = 'Jason' THEN 1 ELSE 0
END
)