I am using Geoserver 2.10.1 and I already developed GIS application using standalone version. Now how do I convert existing geoserver standalone app to web archive so that I copy that web archive to tomcat server which is running other web applications under development which will access the services provided by geoserver web archive?
You need to locate and backup your data directory. Look under the settings menu if you don't know where it is. It's all pretty well covered in the documentation.
edit WEB-INF/web.xml
look for set the param-value to point to the old data directory
... GEOSERVER_DATA_DIR /var/lib/geoserver_data ...
Or set a GEOSERVER_DATA_DIR
environmental variable.