Can someone help me as to how I can open a PPS
Normally you would open LibreOffice Impress and then choose the menu File → Open and navigate to your PPS-file. Actually you should be able to do this from most of the other LibreOffice modules.
(It’s up to your operating system what happens when you double-click on a file. If you want to associate PPS-files with LO Impress, you’d have to do that in the operating system or in a file-manager.)
If you want to open the pps file for editing - change the extension from pps to ppt and then open it.
Open(the presentation starts)
Edit the presentation