di.xml
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="\Magento\Quote\Model\CouponManagement" type="Test\TestOverride\Model\CouponManagement" />
</config>
type="Test\TestOverride\Model\CouponManagement"
write your vendor and module name
CouponManagement.php
Test\TestOverride\Model\CouponManagement.php