Labels are separate objects in Perforce and contain a list of files at specific revisions. You can use a label to revert to previous release (See Syncing with a Label for more information).
The following instructions apply to the Windows client (P4WIN) only.
View to that needed by the project being labelled (See Using Wildcards and Excluding Files for examples). Add/Replace Files Listed in Label… from the popup menu. Perforce will open a dialog offering to replace the contents of the label. Ok to add the files.