Hi there,
It's in the file cart.php in administrator/components/com_hikashop/helpers. The lines you are looking for are:
<table>
<tr>
<td rowspan="2">
<input id="hikashop_product_quantity_field_'.$i.'" type="text" value="'.JRequest::getInt('quantity',$min_quantity).'" class="hikashop_product_quantity_field" name="quantity" onchange="hikashopCheckQuantityChange(\'hikashop_product_quantity_field_'.$i.'\','.$max_quantity.','.$min_quantity.');" />
</td>
<td>
<a id="hikashop_product_quantity_field_change_plus_'.$i.'" class="hikashop_product_quantity_field_change_plus hikashop_product_quantity_field_change" href="#" onclick="return hikashopQuantityChange(\'hikashop_product_quantity_field_'.$i.'\',1,'.$max_quantity.','.$min_quantity.');">+</a>
</td>
<td rowspan="2">
'.$html.'
</td>
</tr>
<tr>
<td>
<a id="hikashop_product_quantity_field_change_minus_'.$i.'" class="hikashop_product_quantity_field_change_minus hikashop_product_quantity_field_change" href="#" onclick="return hikashopQuantityChange(\'hikashop_product_quantity_field_'.$i.'\',0,'.$max_quantity.','.$min_quantity.');">–</a>
</td>
</tr>
</table>
I would however suggest you to try to use css to do what you want instead of editing this code since any modification would not be conserved when updating.