Go to Layouts tab in template settings, choose the theme you are using, click on the edit icon. You’ll see something like
...
<blocks name="middle" colwidth="20" markup="3" fixheight="1">
<block name="content-mass-top" style="raw">content-mass-top</block>
<block name="content-top">content-top</block>
<block name="inset1" width="20">left</block>
<block name="right1" width="30">right</block>
</blocks>
...
change to
...
<blocks name="middle" colwidth="20" markup="3" fixheight="1">
<block name="content-mass-top" style="raw">content-mass-top</block>
<block name="content-top">content-top</block>
<block name="inset1" width="20">left</block>
<block name="right1" width="20">right</block>
</blocks>
...