How to Change Your Blogger Template:-
Blogger has introduced a new Design feature recently. There are many designs now available for your template. They are located in your Design tab, under "Template Designer New!" If you choose to make changes, you need to back up your current template so you don't lose it (there is a huge orange button at the top to remind you of this). You can now adjust column width, number of columns, and many other features without adjusting HTML. It will show you a preview, and then you have to click "Apply to Blog" before it's official:
You can also STILL CHOOSE from the old designs previously offered in Blogger! (Minima is the most common of these, and the easiest to adjust for widening width and adding columns if you prefer HTML hacks and tweaks.) These layouts are located in your Design tab under "Edit HTML." Scroll down until you see "Old Templates" in orange just below the HTML field. You can click the blue link that says "Select Layout Template." The Classic Template (the very original Blogger themes) are also located just below that: