/skin/common_files/modules/One_Page_Checkout/opc_summary.tpl
<div class="terms_n_conditions">
<label for="accept_terms">
<input type="checkbox" name="accept_terms" id="accept_terms" value="Y" />
{$lng.txt_terms_and_conditions_note|substitute:"terms_url":"`$xcart_web_dir`/pages.php?alias=conditions":"privacy_url":"`$xcart_web_dir`/pages.php?alias=business"}
</label>
</div>
to
<div class="terms_n_conditions">
<label for="accept_terms">
<input type="checkbox" name="accept_terms" id="accept_terms" value="Y" checked />
{$lng.txt_terms_and_conditions_note|substitute:"terms_url":"`$xcart_web_dir`/pages.php?alias=conditions":"privacy_url":"`$xcart_web_dir`/pages.php?alias=business"}
</label>
</div>
Add
checked to
<input type="checkbox" name="accept_terms" id="accept_terms" value="Y" />
to make
<input type="checkbox" name="accept_terms" id="accept_terms" value="Y" checked />
Save, uoload, and clear template