Hi All,
I got the solution for this..I am able to fix it on my own after few trial and errors. 
finally it works.
Here you go with steps :
- Open the workflow and save your changes to workflow action sets
- Now select the drop down arrow of action set 'save as snippet ' option --> give a unique name to identify it.
3. Now in another tab open the URL http://(SiteUrl)/NintexSnippets/Forms/AllItems.aspx
4. Here you can see folders created with some unique IDs for each user (as shown below). Open the folder that has been created on your name (as marked red in below).
Now in another tab open the URL http://(Destination-SiteUrl)/NintexSnippets/Forms/AllItems.aspx
6. Here if you don't see any folder has been created on your name (same as shown in above), open the workflow where you need to deploy your changes save any action set as snippet inside the workflow.
7. Now if you see http://(Destination-SiteUrl)/NintexSnippets/Forms/AllItems.aspx folder will be created on your name.
8. Now Open the Folder (created on your name) select "Documents (from the tope menu ribbon)--> Upload document --> browse to the http://(SiteUrl)/NintexSnippets/Forms/AllItems.aspx --> select the snippet that you want to upload to the destination workflow."
9. To get the "WorkflowPartId" value open the snippet (that you are deploying on to other site) with internet explorer --> go to the last line of the page --> select the value inside the <ID> tag (as shown in below figure) copy and paste it in "WorkflowPartId".
10. Now select save/check in to save the snippet.
11. To use this snippet on destination workflow, open the destination workflow --> you will be able to see the "My Snippets" tab on actions panel --> just drag and drop to the workflow where you want to add it.
***Note: double check the values of actions those are added inside the workflow as these may be reset to blank values if any list/library has been selected inside it. ***
I can't find this option on O365 SharePoint online using Nintex Workflow.
