have iphone 4s and need to update ios how

i have been given a old iphone 4s im a dummy to all this. im trying to install messenger on it but it says i have to update my ios to above to do this. I went on itunes as instructed and plugged into my computer but do not get the screen as indicated with menu etc and to choose my phone. i have already checked for updates on my phone as well manually. please is there anyone who can help a complete dummy with simple instructions on how to do this thank you