Ah now i understand whatyou mean. I thought 'thin' a polygon as in make it smaller. So i gave you how to edit it. You want to open the soldermask on it.
here is a trick. Select polygon. Edit - copy - click on a pad of a component . Any component but remeber where you clicked. Now, in the inspector : change the layer of the polygon to top solder ask. If it asks to repour, say no.
Now do edit-paste and click again on the pad of the component you picked when you did edit copy.
Basically what i am doing is storing a copy of the polygon i. The clipboard , using a component pad as origin. I use a component pad as it is a point and altium will 'snap' to the exact centerof the pad.
Then i alter the layer of the existing polygon, telling altium not to redraw it.
After that i paste my copy from the clipboard back in , using that component pad as the placement origin.
This trick works with anything you want to move layers.