Assuming you are using a default installation, the quickest solution is to copy webapps/share/WEB-INF/classes/alfresco/site-data/presets/presets.xml
to tomcat/shared/classes/alfresco/web-extension/site-data/presets/presets.xml
look up the user-dashboard preset and edit (the copy) so it reads:
<!-- Well known preset used to generate the default User dashboard -->
<preset id="user-dashboard">
<components>
<!-- title -->
<component>
<scope>page</scope>
<region-id>title</region-id>
<source-id>user/${userid}/dashboard</source-id>
<url>/components/title/user-dashboard-title</url>
</component>
<!-- dashboard components -->
<component>
<scope>page</scope>
<region-id>full-width-dashlet</region-id>
<source-id>user/${userid}/dashboard</source-id>
<url>/components/dashlets/dynamic-welcome</url>
<properties>
<dashboardType>user</dashboardType>
</properties>
</component>
<component>
<scope>page</scope>
<region-id>component-1-1</region-id>
<source-id>user/${userid}/dashboard</source-id>
<url>/components/dashlets/my-tasks</url>
</component>
<component>
<scope>page</scope>
<region-id>component-1-2</region-id>
<source-id>user/${userid}/dashboard</source-id>
<url>/components/dashlets/my-sites</url>
</component>
<component>
<scope>page</scope>
<region-id>component-2-1</region-id>
<source-id>user/${userid}/dashboard</source-id>
<url>/components/dashlets/my-activities</url>
</component>
<component>
<scope>page</scope>
<region-id>component-2-2</region-id>
<source-id>user/${userid}/dashboard</source-id>
<url>/components/dashlets/my-documents</url>
<properties>
<height>240</height>
</properties>
</component>
</components>
<pages>
<page id="user/${userid}/dashboard">
<title>User Dashboard</title>
<title-id>page.userDashboard.title</title-id>
<description>Users dashboard page</description>
<description-id>page.userDashboard.description</description-id>
<template-instance>dashboard-2-columns-wide-right</template-instance>
<authentication>user</authentication>
</page>
</pages>
</preset>
Be clear that this will affect only users visiting share the first time after you made the change.