First, your closing parenthesis is in the wrong place.
insert into T1 (SR_NO, AH, Model, Price, Qty, ID)
Values('" + textBox1.Text +
"', '" + textBox2.Text +
"', '" + textBox3.Text +
"', '" + textBox4.Text +
"', '" + textBox5.Text +
"', " 1) where ID = 1
Second, you should be using parameterized queries, which would change your query to look like this:
insert into T1 (SR_NO, AH, Model, Price, Qty, ID)
Values(@srNo, @ah, @model, @price, @qty, @id) where ID = 1
Third, you don't need a where clause in an insert statement. Why are you assigning an ID yourself? You can make the database do that for you when a record is inserted. This (along with parameterized queries) would make your query look like this:
insert into T1 (SR_NO, AH, Model, Price, Qty)
Values(@srNo, @ah, @model, @price, @qty)
Finally, follow industry practice when writing SQL queries. It will generally help you read the query better.
INSERT INTO [database_name].[dbo].[T1] ([SR_NO], [AH], [Model], [Price], [Qty])
VALUES (@srNo, @ah, @model, @price, @qty)