To enter in system library , click on finder > Go > Computer > Macintosh HD > Library > preferences .
In system library a Mac user must not delete any .plist files as it can corrupt the operating system .
To enter in user library , click on finder > Go > hold option key > click on library > in preferences folder you will find variety of .plists .
Deleting the .plist for related application may or may not work , you have to only consult apple support and take views from them which plist are to be removed and in what circumstances .
Just an example if mail application is corrupted if you delete com.apple.mail.plist , possibilities are there mail data could be lost .
Deleting of plist is not a guaranteed solution that will normalise the corrupted application .