Hi Corrine,
thanks a lot for the inputs.
For to change existing .CSS file and uploading with new changes, below steps will helpful.
first in browser by focusing the respective element and then Inspect element in mouse right click then at right side it displays styles, the font size, color etc. so there we can change the font-size, margin-top as per the need so that it will show the results immediately in browser(For to get idea on what changes need to apply on .CSS file). Then by right click the respective .CSS file will give options like Upload/Download--> Download, Download with Lock, Upload and Replace.
So, with Download with lock we will save to desktop and do the respective changes. And now in the respective path shown in inspect elements right side shown for styles etc/from where we downloaded .css, we will try to upload the changed file. Before upload, to make sure the file changes don't have syntax errors, please check in online tools for .css file check in google, then try to right click on .css file in SE80 and chose option Upload and Replace. Thats it.
Now Delete the browser cache and try to login again in Web UI, now changes got effected in UI.
Thanks,
Vij