ITSM is IT Service Management portal for the WOG, this is where your domains (the URLs of your website is managed). Typically only your IT Department has access to ITSM.
If you have access to ITSM but still not sure how it works, you can refer to this quick guide on making A and CNAME records changes.
To access ITSM, you go to https://itsm.sgnet.gov.sg
You can only access ITSM on GSIB laptops.
Step 1 - Go to your ITSM (IT Service Management) portal, select Step 2 - Click on External and you'll see a list of domains you own. Step 3 - Click on the domain of your site that you're migrating. You will see a list of records of your domain. Step 4 - To amend your A - Record, select the tab that says "A". You will see where the A record is pointing to.
Step 5 - Look for the A-record which was pointing to the IP address that the previous service hosting your website and click Modify (the orange button)
To find out where your website is previously pointing to, you can go to Google's Dig ToolBox.
Step 6 - Change the entry record to the IP address given by Isomer team. and make sure that TTL is set to 5 mins. If your TTL is longer, the swing in records will take longer.
Click Modify A Record when you're done
How to change the CNAME record?
To change your CNAME record, follow the same step but instead of "A", select the "CNAME" tab.