Hi Chuck
You firstly need to go to your Adsense account and make some ads which are the size you would like, or maybe just one which is the correct size which you want to place a few times in side bar and within pages.
You then go to wp-admin > Appearance > Widgets > Left Side Bar (or Right Side Bar – whichever is applicable). Click on the small arrow to the right of the side bar so that it expands and you will see a PHP Code box which houses your Adsense code. Simply replace that code with your new code.
If you want to add more ads (Adsense or others) grab another PHP Code box from the centre and drag it into the side bar in whatever position you would like. Paste the code again.
Penny