Hi,
Go in the menu Display>Views. For the backend, in the file product / form (and product / variant for the variant edit form), you can find that code:
<div class="input-prepend"><?php
echo str_replace('#MYTEXT#', '<span class="add-on"><i class="hk-icon-14 icon-14-length"></i></span><input size="10" style="width:50px" type="text" id="data_product__product_length" name="data[product][product_length]" value="' . $this->escape(@$this->product->product_length) . '"/>', hikashop_tooltip(JText::_('PRODUCT_LENGTH'), '', '', '#MYTEXT#', '', 0));
?></div>
<div class="input-prepend"><?php
echo str_replace('#MYTEXT#', '<span class="add-on"><i class="hk-icon-14 icon-14-width"></i></span><input size="10" style="width:50px" type="text" id="data_product__product_width" name="data[product][product_width]" value="' . $this->escape(@$this->product->product_width) . '"/>', hikashop_tooltip(JText::_('PRODUCT_WIDTH'), '', '', '#MYTEXT#', '', 0));
?></div>
<div class="input-prepend"><?php
echo str_replace('#MYTEXT#', '<span class="add-on"><i class="hk-icon-14 icon-14-height"></i></span><input size="10" style="width:50px" type="text" id="data_product__product_height" name="data[product][product_height]" value="' . $this->escape(@$this->product->product_height) . '"/>', hikashop_tooltip(JText::_('PRODUCT_HEIGHT'), '', '', '#MYTEXT#', '', 0));
?></div>
So you can change the order of the divs there to change the order.
For the frontend, it's even easier. Edit the view file product / show_block_dimensions and you can use the drag & drop tool to swap them.