How can i change label text in main form, when a button in second form is clicked?

I am trying to make a tic tac toe game where you choose user names before you can play. Upon clicking "choose users" a second form shows. After typing names and choosing X or O, the OK button is to be clicked and once it is, I would like two labels in the main form to change their text. How can I do this? It should be noted that the main for stays open during this time so changing the text on loading isnt possible.