Can you be a bit more specific here?

If you are talking about "rest api versioning", well, with 6.2 you are more or less out of luck. You probably can implement some workarounds, if you have only minor changes, but each time, you execute Service Builder, the old interface is just replaced with the new one.

Yes are are creating rest webservice in Liferay 6.2 and these services are being consumed in Mobile App .Let say we have one service showEmployee() and further if we made change in these service then we want our client part should  not break.

In 6.2 you are more or less out of luck.
There are really ugly hacks I could think of to solve your problem, but IMHO your best option is to just add new functions and document them. If you have just a few functions, you could add a showEmployee_v2() method, as ugly as it is, and work with that.
Of course, this will only work for simpler, only additive changes, but in 6.2 your options are limited.