As Thomas and Tore have said, DOW() is the key. This code seems to do what you want:

LOCAL ldDate as Date, lni as Integer, lnSno as Integer  
ldDate = {^2022-05-02} && starting date
lnSno = 5 && so sno is not tied to loop counter
FOR lni = 5 TO 16 && per range in your example
   IF DOW( ldDate) = 1 && Sunday
   ELSE
      INSERT into date_step(sno,datex,subject) VALUES (lnSno,ldDate,'science') && start from sno = 5
      lnSno = lnSno + 1
   ENDIF && DOW( ldDate) = 1 
   ldDate = ldDate + 1
ENDFOR && lni = 5 to 16

Result:

 5 02/05/2022 science 
 6 03/05/2022 science 
 7 04/05/2022 science 
 8 05/05/2022 science 
 9 06/05/2022 science 
10 07/05/2022 science 
11 09/05/2022 science 
12 10/05/2022 science 
13 11/05/2022 science 
14 12/05/2022 science 
15 13/05/2022 science 

>

>CREATE table date_step (sno n(6),datex date, subject C(20))
>SET DATE TO DMY
>
>INSERT into date_step(sno,datex,subject) VALUES (1,{^2022-05-02},'science')
>INSERT into date_step(sno,datex,subject) VALUES (2,{^2022-05-03},'science')
>INSERT into date_step(sno,datex,subject) VALUES (3,{^2022-05-04},'science')
>INSERT into date_step(sno,datex,subject) VALUES (4,{^2022-05-05},'science')
>INSERT into date_step(sno,datex,subject) VALUES (5,{^2022-05-06},'science')
>INSERT into date_step(sno,datex,subject) VALUES (6,{^2022-05-07},'science')
>INSERT into date_step(sno,datex,subject) VALUES (7,{^2022-05-09},'science')
>INSERT into date_step(sno,datex,subject) VALUES (8,{^2022-05-10},'science')
>INSERT into date_step(sno,datex,subject) VALUES (9,{^2022-05-11},'science')
>INSERT into date_step(sno,datex,subject) VALUES (10,{^2022-05-11},'science')
>
>Now I want to update record like this. 
>start from new date and skip Sunday
>
>
>INSERT into date_step(sno,datex,subject) VALUES (5,{^2022-05-02},'science')&& start from 5
>INSERT into date_step(sno,datex,subject) VALUES (6,{^2022-05-03},'science')
>INSERT into date_step(sno,datex,subject) VALUES (7,{^2022-05-04},'science')
>INSERT into date_step(sno,datex,subject) VALUES (9,{^2022-05-05},'science')&& skip sunday
>INSERT into date_step(sno,datex,subject) VALUES (10,{^2022-05-06},'science')
>INSERT into date_step(sno,datex,subject) VALUES (11,{^2022-05-07},'science')
>INSERT into date_step(sno,datex,subject) VALUES (12,{^2022-05-09},'science')
>INSERT into date_step(sno,datex,subject) VALUES (13,{^2022-05-10},'science')
>INSERT into date_step(sno,datex,subject) VALUES (14,{^2022-05-11},'science')
>INSERT into date_step(sno,datex,subject) VALUES (16,{^2022-05-11},'science')&& skip sunday always skip sunday
>...................................................
>
>