On the home page in the top right corner, to the right of your profile picture there will be a triangle, you need to click on it.
When you click on the triangle it should look like the attached image.
Click 'Admin Tools' then under 'Appreciation' click 'Add Cash Giving'
When adding cash, the number you enter into 'Add Amount' will add on to what that person can already give.
For example, If that person can give $50 and you want them to be able to give $60, the number you enter into would be $10. This is shown in the image below.