If the user has purcahsed the product already, show a links to download each file associated with the product instead of an Add To Cart Button
<?php
/*
 * Plugin Name: Easy Digital Downloads - Replace Add to Cart with Download
 * Description: If the user has purcahsed the product already, show a links to download each file associated with the product instead of an Add To Cart Button
 * Author: Easy Digital Downloads
 * Author URI: https://easydigitaldownloads.com/
 * Version: 1.1
 */

function ck_edd_user_download_button( $purchase_form, $args ) {
	global $edd_options;

	if ( !is_user_logged_in() ) {
		return $purchase_form;
	}

	$download_id = (string)$args['download_id'];
	$current_user_id = get_current_user_id();
	// If the user has purchased this item, itterate through their purchases to get the specific
	// purchase data and pull out the key and email associated with it. This is necessary for the
	// generation of the download link
	if ( edd_has_user_purchased( $current_user_id, $download_id, $variable_price_id = null ) ) {
		$user_purchases = edd_get_users_purchases( $current_user_id, -1, false, 'complete' );
		foreach ( $user_purchases as $purchase ) {
			$cart_items    = edd_get_payment_meta_cart_details( $purchase->ID );
			$item_ids = wp_list_pluck( $cart_items, 'id' );
			if ( in_array( $download_id, $item_ids ) ) {
				$email       = edd_get_payment_user_email( $purchase->ID );
				$payment_key = edd_get_payment_key( $purchase->ID );
			}
		}

		$download_ids = array();
		if ( edd_is_bundled_product( $download_id ) ) {
			$download_ids = edd_get_bundled_products( $download_id );
		} else {
			$download_ids[] = $download_id;
		}

		// Setup the style and colors associated with the settings
		$style             = isset( $edd_options['button_style'] )   ? $edd_options['button_style'] : 'button';
		$color             = isset( $edd_options['checkout_color'] ) ? $edd_options['checkout_color'] : 'blue';
		$new_purchase_form = '';

		foreach ( $download_ids as $item ) {
	 		// Attempt to get the file data associated with this download
			$download_data = edd_get_download_files( $item, null );

			if ( $download_data ) {
				foreach ( $download_data as $filekey => $file ) {
	 				// Generate the file URL and then make a link to it
					$file_url = edd_get_download_file_url( $payment_key, $email, $filekey, $item, null );
					$new_purchase_form .= '<a href="' . $file_url . '" class="' . $style . ' ' . $color . ' edd-submit"><span class="edd-add-to-cart-label">Download ' . $file['name'] . '</span></a>&nbsp;';
				}
			}

			// As long as we ended up with links to show, use them.
			if ( !empty( $new_purchase_form ) ) {
				$purchase_form = '<h4>' . __( 'You already own this product. Download it now:', 'edd' ) . '</h4>' . $new_purchase_form;
			}
		}
	}

	return $purchase_form;

}
add_filter( 'edd_purchase_download_form', 'ck_edd_user_download_button', 10, 2 );
To implement this snippet on your site, create a brand new, empty WordPress plugin and paste the above code inside.