Hi, once you are inside the button creation page of your PayPal profile go to the >> "Step 1: Choose a button type and enter your payment details", here select the option "Donors contribute a fixed amount." and insert the fixed amount.
To enter that page:
- log in into your PayPal account - selct Profile - select My selling preferences (Alternatively click on this direct link https://www.paypal.com/businessprofile/mytools ) - click UPDATE next to PayPal buttons - next to your existing button click the combo "Action" and select "Edit Code"
If you have an already created button you can also just add to your HTML button code the HTML line with an amount like this one: