If you intend to do any editing of the template of your New Blogger template, a wise thing to do is first backup your current template PLUS edit your Page Elements one by one, pasting the content into Notepad files and saving them together with your backup template. This has been made very easy for you. Just sign into the Dashboard, and in the section for the relevant blog, click LAYOUT, then click the TEMPLATE tab. Click the EDIT HTML sub-tab below that. This will open your template editor window where you can make changes to your template, but before you make any changes to the template, it is always wise to BACKUP the template first.
To backup the template, click the "Download full template" link and save the backup template into a suitable folder with a filename like blognameDate (substitude blogname with the name of your blog, plus any information that will help you pick out the template you want such as information on changes you plan to make, etc.) or other system of naming which will enable you to identify the backup template easily.
If you have been adding Page Elements, you should also edit the Page Elements one by one and copy its content into Notepad files, save them into the same folder that you save your backup template. This is because when you upload your backup template, there is a possibility that some Page Elements will not be saved. If you have backed up these Page Elements, all you need to do is to put them back.
Backing up Page Elements have been described in more details in Back New Blogger Page Elements (click BACK button to get back to this page).
After you have done that, you can edit the template in the template editor window with peace of mind, knowing if you mess it up, you can just upload your backup template plus put back in any Page Elements that got deleted in the process, and you will be back to your former state.
Regarding editing of the template, how you do it will depend on what you want to do. The simplest would be to edit the template without expanding the widget templates. The template will be easiest to read as it leave out a lot of parts of the template. However, sometimes this is not good enough, you will have to tick the "Expand widget template" to get access to the rest of the template. Whichever way you do will depend on what you want done. Follow the instruction.