i came across this article and i followed the steps

https://daviddrever.com/2018/04/microsoft-powerapps-updating-a-data-card-from-a-button/

Here is what i did:

Step1- in the OnVisible property of Screen1 i have this: Set(AmountVar,SharePointIntegration.Selected.Amount)

Step2- The Value property of DataCardValue3 i set to AmountVar (the new variable which i created in Step1)

Step3- The OnChange property of DataCardValue3 i have this: UpdateContext({AmountVar:3})

thats it

now when i change the amount it gets set to 3