It is a good question and the very simple answer is: you cannot. Blogger does not support a last modified header.
This page lists all the data points available for parsing and you will soon find that the dating options are limited. It simply isn't possible.
If you are wanting to add the relevant markup to appease the Structured Data Tool then you can tell it to use the original post date by adding or editing the template:
<abbr class='published' expr:content='data:post.timestampISO8601' itemprop='datePublished dateModified'><data:post.timestamp/></abbr>
The exact code you want to use depends on how your template is laid out. The above works for me.
In your specific case, if you wish to use a
<meta> tag then you'll want to use:
<meta http-equiv="last-modified" expr:content="data:post.timestampISO8601" />
Alternatively you can add the data manually when you are editing the post. You'll need to use the HTML editor and add a relevant element. It's up to you whether you want it to show on the page or not.
Last updated: <div content='CCYYMMDDTHHMMSSZ' itemprop='dateModified'>March 2016</div>
<meta content='CCYYMMDDTHHMMSSZ' itemprop='dateModified'/>
I prefer the latter but you'll see some bloggers do make their modified time visible. Depends on the nature of your blog and personal preference.