How to import/upload a .docx file in RRC so that it opens not with an external editor? Also, how to make changes and save the same?

Depending on which version of RRC you're using, the following links should be helpful:

RRC 3.0.1.x: http://pic.dhe.ibm.com/infocenter/clmhelp/v3r0m1/index.jsp?topic=%2Fcom.ibm.rational.rrm.help.doc%2Ftopics%2Ft_import_word.html

RRC 4.0.x: http://pic.dhe.ibm.com/infocenter/clmhelp/v4r0/index.jsp?topic=%2Fcom.ibm.rational.rrm.help.doc%2Ftopics%2Ft_import_word.html

Click on the Artifacts dropdown menu (where you can browse and create artifacts) and select the 'Import' option. You will see five options. If you select the 'Import requirements from within a text document' option, you can specify artifact types for the different kinds of found requirements. If you want to import your .docx file and see it just as one single rich artifact, then select the 'Import a text document and convert to a rich text artifact' option.