The "KG" and "LBS" definitions are nothing more than "LABELS". And you change the definition in LANGUAGE FILES.
In the store-front, the reference is in the main english.php file, and as you should be using a CUSTOM TEMPLATE, this file is in:
includes/languages/YOUR_CUSTOM-TEMPLATE/english.php
If you do not have either that folder in that location, nor the english.php file inside it, then you must CREATE that folder - sometimes called the OVER-RIDE folder - on your server. That folder MUST bear the SAME NAME as your custom template folder.
Then, you put a COPY of english.php inside that folder.
You do this by collecting a COPY of the core file via FTP. So you FTP a copy to your home computer. Then, you FTP that copy to the OVER-RIDE folder you created.
You NEVER edit the CORE file... only the COPY that now sits in your OVER-RIDE folder.
Then.., to change LBS to KG, open that copied english.php file for editing in a PLAIN TEXT EDITOR such as CRIMSON EDITOR
and find:
PHP Code:
define('TEXT_PRODUCT_WEIGHT_UNIT','Lbs');
Change to:
PHP Code:
define('TEXT_PRODUCT_WEIGHT_UNIT','Kg');
--------------------------------
For the ADMIN area, things are a bit different, because there is no OVER_RIDE system in admin... you edit the CORE FILE.
So... first thing to do is to start a formal DIARY of the edits you make, so you have a chronology of what you have done. Very useful for debugging and later upgrades.
The same principle applies to the english.php file in admin
admin/includes/languages/english.php
PHP Code:
define('TEXT_PRODUCT_WEIGHT_UNIT','Lbs');
BE CAREFUL NOT TO CONFUSE THE TWO english.php FILES... THE ADMIN VERSION IS DIFFERENT FROM THE STORE-FRONT ONE.
-----------------------------------------
That sorts out the issue of the LABEL applied to the weight variables.
The REST of the process is purely arithmetic, because the NUMBER VALUES you now key in will be processed mathematically by the programming.
As your WEIGHT LABEL is now Kg, the VALUES you put into the weight field for each product must be represented in proper decimalisation of Kgs.
ONE KG will be input simply as 1
TWO HUNDRED GRAMS will go in as 0.2
ONE GRAM will go in as 0.001
...and so on. If you didn;t pay attention in maths class, ask your 10-year-old son/daughter to show you how to decimalise KG and fractions of KG... 
------------------------------------------
So... if your shipping relies on WEIGHT values, the first step is to make sure all your products have correct weights... and it is just a NUMBER you enter into the weight field... do not enter "kg"... JUST the number.
Bookmarks