First you need to go into theme-admin.php and in function new_wpestate_theme_admin_general_settings you have

array( 'name' => __('feet','wpestate'), 'unit' => __('ft','wpestate'), 'is_square' => 0 ),
array( 'name' => __('meters','wpestate'), 'unit' => __('m','wpestate'), 'is_square' => 0 ),
array( 'name' => __('acres','wpestate'), 'unit' => __('ac','wpestate'), 'is_square' => 1 ),
array( 'name' => __('yards','wpestate'), 'unit' => __('yd','wpestate'), 'is_square' => 0 ),
array( 'name' => __('hectares','wpestate'), 'unit' => __('ha','wpestate'), 'is_square' => 1 )

add another element in that array

After that go into theme admin , and pick the new measurement unit you just added.

The defaul units come with a 2 at the end . If you want to remove this you will need to remove the string  ‘ <sup>2</sup>’  in the following places

compare_listings.php – lines 54 and 59

mapfunctions.js – lines 375 and 377

help_functions.php lines 351 and 353

listing_functions.php lines 48, 265, 271

theme-admin.pphp line 1276

theme-slider.php line 113 and 115

property-unit.php line 222

property_details line 18 and 23

A line like this one

print ' '.$measure_sys.'<sup>2</sup> '.__(' (*only numbers)','wpestate');

should become

print ‘ ‘.$measure_sys.”.__(‘ (*only numbers)’,’wpestate’);