I am working on GeoServer 2.13 version. When I am working with WFS-T using OpenLayers I am getting WFS version 1.1.0 request, but my GeoServer accepts only WFS 2.0 requests.
How should I change the WFS default version to 1.1.0 in GeoServer?
Your geoserver will accept any version of the WFS specification (except currently 3.0 which is still experimental). It is up to the client to say which version of the spec it would like to use.
There is a complex version negotiation mechanism if your client doesn't specify a which version it wants, this would usually be version 2.0.