Keeping Track Of Inventory For A Company - How To Update Variables

My program is supposed to be used to keep track of inventory for a company. The user is prompted with a menu that asks them which item they want to update and then gives them another menu that allows them to buy, sell, or change the price of the items. For this, I need to have the variable values to change (based on the input of the user), because they are initially set to specific numbers. How would I do this?

Here's the part of my code that is relevant to this question:

balance = 4000.00;
lampQuantity = 400;
lampPrice = 10;
chairQuantity = 250;
chairPrice = 20.00;
deskQuantity = 300;
deskPrice = 100.00;
  
System.out.println("Current Balance: $" + balance);
System.out.print("1. Lamps " + lampQuantity);
System.out.println(" at $" + lampPrice);
System.out.print("2. Chairs " + chairQuantity);
System.out.println(" at $" + chairPrice);

[Code] .....