Place this in your remotes tab (alt-r twice):
Code:
; $1 = text, $2 = substring, $3 = occurrence number, $4 = replacement
; example: $onesubst(ffabcabdabeabf,ab,3,__) returns ffabcabd__eabf
alias onesubst {
  var %3 = $3 - 1, %2 = $+(\Q,$replacecs($2,\E,\E\E\Q),\E)
  var %re = ^((?:.*? $+ %2 $+ ){ $+ %3 $+ }.*?) $+ %2
  var %res, %4 = $replace($4,\,\\), %u = $regsub($1,%re,\1 $+ $4, %res)
  echo -s 3: %3 ,2: %2 ,4: %4 ,re: %re ,u: %u
  return %res
}

//Echo -s $onesubst(testabtestabtestab,ab,2,ing)
testabtestingtestab