How to replace first 5 digit with * and remaining number will same ?

This will achieve what you've expressed:

fn!concat("XXXXX", fn!right(ri!myNumber, 5))

Note: it's best practice to encapsulate literals in constants so that a) their value can be maintained independently of the code where they're used and b) so that, by giving the constant a meaningful name, the code becomes more readable. e.g. instead of "XXXXX" you may have cons!PROJECT_PREFIX_TELEPHONE_NUMBER_MASKING_VALUE (or whatever is meaningful in your context). And now that you have the value in a constant you could interrogate that constant for it's length and use that value instead of the literal 5, so if the masking string in your constant ever changes in value or length the code will continue to work:

fn!concat(
  cons!PROJECT_PREFIX_TELEPHONE_NUMBER_MASKING_VALUE,
  fn!right(
    ri!myNumber,
    fn!len(
      cons!PROJECT_PREFIX_TELEPHONE_NUMBER_MASKING_VALUE
    )
  )
)

This will achieve what you've expressed:

fn!concat("XXXXX", fn!right(ri!myNumber, 5))

Note: it's best practice to encapsulate literals in constants so that a) their value can be maintained independently of the code where they're used and b) so that, by giving the constant a meaningful name, the code becomes more readable. e.g. instead of "XXXXX" you may have cons!PROJECT_PREFIX_TELEPHONE_NUMBER_MASKING_VALUE (or whatever is meaningful in your context). And now that you have the value in a constant you could interrogate that constant for it's length and use that value instead of the literal 5, so if the masking string in your constant ever changes in value or length the code will continue to work:

fn!concat(
  cons!PROJECT_PREFIX_TELEPHONE_NUMBER_MASKING_VALUE,
  fn!right(
    ri!myNumber,
    fn!len(
      cons!PROJECT_PREFIX_TELEPHONE_NUMBER_MASKING_VALUE
    )
  )
)