In the window in which you see your portfolio, you have a choice to select one of the securities from your portfolio.
A new window is shown, and in it, you can enter an imaginary price, and have the value of the security calculated, as if the security had the price you entered.
You simply enter the price in the window that pops up, press the "calculate" button, and the value will then be shown, together with a calculated profit/loss value.
If the currently selected security from the portfolio, is traded in a foreign currency, the price entered should be the price in the local currency. Another field will become visible, at in that field, the currency exchange rate can be entered. If there are known currency rates for the country, the latest rate will be shown in the field. The profit/loss calculation will then automatically show the result in you own base currency.
This simple "what-if" calculation can be accompanied by the extended "what-if" calculation, that is a part of the "Alarms & Watches" section of Amateur Invest. See more about extended "What-if" here.