when i replace 'ت' sql change all 'ت' and 'ة'
declare @name as nvarchar(max);
set @name = N'ةاتی تو را است ت';
declare @char1 as nvarchar;
set @char1 = N'ت';
declare @char2 as nvarchar;
set @char2 = N'ب';
select replace(@name, @char1 ,@char2);
Try this
declare @name as nvarchar(max);
set @name = N'ةاتی تو را است ت';
declare @char1 as nvarchar;
set @char1 = N'ت';
declare @char2 as nvarchar;
set @char2 = N'ب';
select replace(@name Collate Persian_100_CI_AI, @char1 ,@char2);
to see all available collations, you may want to change CaseSensitive/Insensitive and/or Accent...
SELECT * FROM ::fn_helpcollations() FH WHERE name LIKE '%Persian%'
It also might be a good idea to google for other questions and results, see Problem with Persian_CI_AI collation And 'ك' character