You can use the following code to change the increment of the donation amount input box. Just add the filter in the functions.php file of your theme. Tip: You can of course also use the free Code Snippets plugin instead.
add_filter('wcdp_donation_amount_decimals', function() {
return 1; //return increment here (e.g. 0.01, 0.1, 1, 2, 5 etc.)
});