How to pass value from textbox in datagridview

I am new at programming (first week on this area) and need some help from you guys. I am developing a small pos where in first form I have a datagridview where the data are showed from sql . The datagridview has those columns (BC, Name, QTY, Price ,Total). The user can edit the qty by typing the qty column on selected row.Now I want to make more easier, the user opens the qty form(second form by clicking the button), where the form I loaded the user can type the qty number on textbox. My question is now how should I pass the value from textbox (2nd form) on datagridview selected row/ qty column (first form) . . Could some help me about this problem.. Thanks to everyone!!

I don't have any code to show to you, cause as I mention above I am new and don't have any experience