Arduino help to filter and modify serial output

Hey hoping someone can help, i have a data logger that outputs its log lines to serial id like to use an arduino to take those log line and spit them back out in a format my satilite modem understands modem is a m138 swarm unit and has redily avalible library but also accepts and logs TD" messeges untill next passover.

Not sure what familiarity you might have with arduino programming, or the exact type of help you might be seeking, here is a rough skeleton of reading and writing a text string.

void setup() {
Serial.begin(9600); // initialize serial communication at 9600 bits per second:

void loop() {
if (Serial.available()) { // check if data is available to read
String inputString = Serial.readStringUntil(ā€˜\nā€™); // read the incoming data as string until ā€˜\nā€™ (newline) character is received
inputString.trim(); // remove any leading/trailing white spaces
Serial.print("The message you sent is: ");
Serial.println(inputString); // print the message in its original format
Serial.print("The message you sent in uppercase is: ");
Serial.println(inputString.toUpperCase()); // print the message in uppercase format