Hi, Yes, you can connect from one Scene to another. There are many ways to do that.
In your case, it might be useful to do this with a signal. This means when you get a ui_touch event you can do your local processing to update the Score and then Emit a ScoreUpdated signal which you can connect to a function in Main Scene to update the score.
More details on signals here
Another way to do that is to have your Player scene reach out and update the Main scene using a nodepath. To do that, Player needs to know the nodepath to reach Main. You can read up about nodepaths in the docs. These can be fragile if you change the design of your node tree, so be careful when designing this :-)
There is an end to end example of that approach here or another one here