Here is a simple tutorial on how to install XML blogger templates in your blog without losing your widgets.
1. Sign in to Blogger dashboard and click on the layout.
2. Here is the most important step. After uploading any new Blogger template, all of the previous widgets like your google adsense ads, text, profile, polls, and etc will disappear. To avoid this, you have to go to Page Elements and then click on Edit on all the widgets and copy the codes into notepad,etc and save it.
3. Next, click on the Edit HTML option.
4. Click on the Download Full Template to download the current template. As a backup.
5. Now, click on the Upload button to upload your template. Remember, Blogger only supports XML format templates.
6. Now, a message appear saying that your widgets are about to be deleted.Click on the Confirm & Save (no problem doing this because we already copied the widget codes in Step 3).
Now you successfully installed the template. So, click on the 'Page elements' page now and add the codes that you copied in step 2 using the 'Add a Page Element' option.