My requirement is whenever in external system , if Template Id changes , then I need to update in SFDC custom setting. Can anyone help me how to generate Webservice class for this so that they can consume and hit the SFDC system and from my end field gets update?
The WSDLS are already there for you
Setup -> Deleop -> API
and download the WSDL based on your requirements:
If you are looking at building your own custom web service (overkill for just a field update) but maybe it is required by your external service start reading: https://developer.salesforce.com/page/Creating_REST_APIs_using_Apex_REST
The Force.com REST API lets you use a simple and lightweight API to access Force.com data, using standard OAuth, together with a choice of data flavors – XML and JSON. The REST API is configured out of the box to suit the basic CRUD (create, read, update and delete) operations of data management.
Apex REST augments this functionality and makes it possible for developers to create your own REST-based web services using Apex. It has all of the advantages of the REST architecture, provides the ability to define custom logic and includes automatic argument/object mapping.
This article provides an introduction and overview of the Apex REST functionality, showing you how to take your first steps in creating REST-based web services in Apex. As Apex REST builds on the Force.com REST implementation itself, the Getting Started with the Force.com REST API article is recommended reading