Copied from the releasenotes in JSPWiki-2.4.87-bin.zip Upgrading from previous releases of 2.4:

• You should be able to just install the JSP files (top-level and the default template), and the set of JAR files.
• In addition, please check web.xml for any additional definitions that may have been added.

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I copied the following files
From: JSPWiki-2.4.87-bin.zip/JSPWiki.war/
to: tomcat/webapps/MyWiki/

• Comment.jsp
• CookieError.jsp
• Delete.jsp
• DeleteGroup.jsp
• Diff.jsp
• Edit.jsp
• EditGroup.jsp
• Error.jsp
• Group.jsp
• Install.jsp
• Login.jsp
• LoginForm.jsp
• Logout.jsp
• LostPassword.jsp
• NewBlogEntry.jsp
• NewGroup.jsp
• PageInfo.jsp
• PageModified.jsp
• Preview.jsp
• Rename.jsp
• rss.jsp
• Search.jsp
• Upload.jsp
• UserPreferences.jsp
• Vote.jsp
• Wiki.jsp

Then I copied all the files including subdirectories and their files
From: JSPWiki-2.4.87-bin.zip/JSPWiki.war/templates/default
To: tomcat/webapps/MyWiki/templates/default

Lastly I copied all the jar files
From: JSPWiki-2.4.87-bin.zip/JSPWiki.war/WEB-INF/lib/
To: tomcat/webapps/MyWiki/WEB-INF/lib/
I used this method I lost all my group permissions. I don't know why. Other than that, the upgrade seems successful.