You can change registered mobile number in HDFC Bank account online by following the steps given below:Step-1: Go to official website of HDFC BankStep-2: Login to HDFC Net Banking PageStep-3: Click on Update Email ID and Landline NumberStep-4: Edit the number you want to changeStep-4: Confirm the number by typing it once againAfter following the steps mentioned in above articles, you can change or update mobile nimber registered in your HDFC Bank Account Online without visiting the bank branch.Your New Mobile Number will be Updated within 24 Hours!!!Point to be NotedIt is true that you can change your mobile number in HDFC Bank Account online still I recommend you to get this done personally by visiting the bank branch. It will eliminate risk( although very rare) of hacking confidential information.Hope this works for you.