You open the web.xml (deployment descriptor) property sheet
from the web.xml's node in the Filesystems Window
Properties
Servlet Spec Version. (Read-Only) Identifies the servlet's
specification version. The version is derived from either the version attribute
of the web-app entry in the web.xml file, or from the DTD version. For
web modules based on specification versions earlier than 2.3, the value
is set to 2.3.