If you are a SiteGround customer, you can manage your domain's A records from your Customer Area > My Account Section > Manage.

Next to your hosting account, click on "Manage". Then you will be taken to a new page with various options. At the bottom of the page you will find "Manage A records". Click on it and you will be guided further into the A records management tool which includes all of your domains.

If you have problems changing your A record, try contacting your web host for assistance. You can always count on Siteground's Support for such problems.