Step 1. Go To Blogger Dashboard >> Design > Edit HTML : and check "Expand Widget Templates" (back up your template)
Step 2. Search (CTRL + F) for the following code:
<b:if cond='data:post.showThreadedComments'>
<b:include data='post' name='threaded_comments'/>
<b:else/>
<b:include data='post' name='comments'/>
</b:if>
</b:if>
<b:if cond='data:blog.pageType == "item"'>
<b:if cond='data:post.showThreadedComments'>
<b:include data='post' name='threaded_comments'/>
<b:else/>
<b:include data='post' name='comments'/>
</b:if>
</b:if>
Step 3. Remove it (you'll find this code twice and replace it twice) and paste this code:
<b:include data='post' name='comments'/>
</b:if>
<b:if cond='data:blog.pageType == "item"'>
<b:include data='post' name='comments'/>
</b:if>
Step 4. Save Template.
That was all!
If you have any questions, feel free to ask.