Maybe you what to use https://github.com/bruderstein/nppPluginManager/releases/tag/v1.4.12 for automatic installation. For me it is working. Otherwise you could check https://github.com/bruderstein/npp-plugins-x64/blob/master/plugins/plugins64.xml#L1228 how it is installed and redo the steps manually.
<download>https://sourceforge.net/projects/npp-plugins/files/XML%20Tools/Xml%20Tools%202.4.9%20Unicode/Xml%20Tools%202.4.9.2%20x64%20Unicode.zip/download</download> <copy from="dependencies\libiconv-2.dll" to="$NPPDIR$\" validate="true"/> <copy from="dependencies\libwinpthread-1.dll" to="$NPPDIR$\" validate="true"/> <copy from="dependencies\libxml2-2.dll" to="$NPPDIR$\" validate="true"/> <copy from="dependencies\libxslt-1.dll" to="$NPPDIR$\" validate="true"/> <copy from="dependencies\zlib1.dll" to="$NPPDIR$\" validate="true"/> <copy from="install.txt" to="$PLUGINDIR$\doc\XmlTools"/> <copy from="XMLTools.dll" to="$PLUGINDIR$\" validate="true"/> <copy from="dependencies\libcurl.dll" to="$NPPDIR$\" validate="true"/>