You can use the following way to cover Post method.
`@isTest
static void testHttpPost() {
// prepare test-data
//As Per Best Practice it is important to instantiate the Rest Context
RestRequest req = new RestRequest();
RestResponse res = new RestResponse();
req.requestURI = ‘/services/apexrest/Something’; //Request URL
req.httpMethod = ‘POST’;
RestContext.request = req;
RestContext.response= res;
Test.startTest();
String actual = null;//HttpClass.updateCustomObject();
Test.stopTest();
System.assertEquals(‘expected value’, actual, ‘Value is incorrect’);
}`