In order to change the "Last modified label" in your XWiki, you will need to override a template inside the skin. More details on this here.
We recommend that you don't edit the original skin (named Default XWiki Skin) to avoid your customization being inadvertently overwritten during a future XWiki upgrade. Instead, you could create a new skin by copying the default one.
To do this, in the Administration menu >> Look & Feel >> Themes, click on the Customize button in the Skin section. Then, in the More actions menu (button with vertical ...), choose to make a copy.
Once the copy has been made, return to the Themes menu, select your copied version of the skin from the drop-down menu and click on the Save button.
For customization, click on the Customize button to edit your own skin. Here you can change the name of the skin (i.e: Copy of Default Skin), and also here we'll override contentheader.vm by following the documentation
As this is an overload, we need to retrieve the original code and add our customization to it. Depending on the version of XWiki that you have, the original code may very. This guide was created by using the current LTS version of XWiki, 14.10.18. The documentation linked above will guide how to check your original code.
Paste this code inside the contentheader.vm Content field:
<div class="row document-header">
<div class="document-info col-xs-12 #if($displayContentMenu)col-md-7#end">
## --------------------------------------------------------
## Display UI Extensions before the title element
## --------------------------------------------------------
#foreach ($uix in $services.uix.getExtensions('org.xwiki.platform.template.title.before'))
$services.rendering.render($uix.execute(), 'xhtml/1.0')
#end
<div id="document-title"><h1>$titleToDisplay</h1></div>
#if (!$doc.isNew())
## Note that this serialization step is mandatory and we cannot count on the UserReferenceConverter here
## because UserReference#toString does not rely on the serializer so it's not the expected format for
## XWiki#getUserName(String).
#set ($authorName = $services.user.serialize($tdoc.authors.originalMetadataAuthor))
<div class="xdocLastModification">
#if (!$doc.isMostRecent())
$services.localization.render(
'core.document.modificationWithVersion',
["<a href='$tdoc.getURL('view', ""rev=$tdoc.version"")'>$tdoc.version</a>",
$xwiki.getUserName($authorName),
$xwiki.formatDate($tdoc.date)
])
#else
<div class='row'>
<div class='col-xs-6'>
Last modified and approved by
$services.localization.render($xwiki.getUserName($authorName)) on
$services.localization.render($xwiki.formatDate($tdoc.date))
</div>
</div>
#end
</div>
#end
</div>
#if($displayContentMenu)
<div class="document-menu col-xs-12 col-md-5">
#template("menus_content.vm")
</div>
#end
</div>
<hr/>
{code}
