Hello firashelou,
You read the rotary switch and use it to determine which page you want. I suggest you maintain a global variable something like:
uint8_t page;
Which holds the page the Nextion is on.
To tell the Nextion to change page you can use something like:
Serial.print ("page");
Serial.print (page);
Serial.write(0xff);
Serial.write(0xff);
Serial.write(0xff);
Note I put that together in a hurry and have not tested it, apologies if it is not quite right.