Hi Teja,
You shall go through the links and SAP notes given below.
http://service.sap.com/instguidesnw74
1751237 - Add. info about the update/upgrade to SAP NetWeaver 7.4 (incl. SPs and SRs)
Regarding Webdynpro for JAVA, you need to go through the JAVADOCS and you need to identify if there is any depricated methods which you were already using in NW 7.3 and you need to replace it will right one as provided in Javadocs for NW 7.4
Thanks,
Mahendran B.