If your website needs more resources than a shared hosting account can provide, or you would like to run an application not suitable for shared hosting, or you simply want a more powerful solution you can easily upgrade to a Semi Dedicated Server. The Semi Dedicated upgrade is available as an extra feature to your hosting plan.
To order Upgrade to Semi Dedicated you need to be logged into your Customer Area first. From the your Customer Area access the Order section.

You will see a list of different extra features that could be added to your account. To proceed with the order, click on the Upgrade to Semi Dedicated Hosting link.

On the next page you can choose the desired plan. You can order it by pressing the Submit button for it.

SiteGround provides three Semi Dedicated plans you can choose from. Every Semi Dedicated server comes with different features. You can review a comparison chart and choose the one that best suit your needs from the link below:
Semi Dedicated Hosting Plans
Note: You can have 1 main website on each Semi Dedicated server and a different number of free addon domain slots depending on the VPS type - 4 with the Advanced Semi Dedicated plan and 9 with the Pro Semi Dedicated plan.