How can i Update 5 rows in CDT at one time?

Do you want to do it during/after loading by a query rule, right before storing in a smartservice, in an interface via button action or Link? 

In an interface via button action

this should do the trick

a!forEach(
    items: {yourCDTarray},
    expression:if(
        fv!Index>1,
        a!update(
            data:fv!item,
            index:"sno",
            value:fv!index+1
        ),
        {}
    )
)

ok Thanks 

Another possible solution to avoid a loop is to use updatecdt() function.

updatecdt(
  ri!yourCDT, /* ca be a local variable or rule input */
  {
    /* Creating an array of first element and {(rest of the elements)+1} */
    SNO: append(ri!yourCDT.SNO[1], ldrop(ri!yourCDT.SNO, 1) + 1)
  }
)

Another possible solution to avoid a loop is to use updatecdt() function.

updatecdt(
  ri!yourCDT, /* ca be a local variable or rule input */
  {
    /* Creating an array of first element and {(rest of the elements)+1} */
    SNO: append(ri!yourCDT.SNO[1], ldrop(ri!yourCDT.SNO, 1) + 1)
  }
)