In Perforce, you can change the filetype of files stored in the depot either by:
The new filetype takes effect when you submit your opened files to the depot.
The Run->Change filetype... menu command is available in both the Path Browser and the File Browser. It brings up a form that lets you enter a new filetype and reopen your opened files using that filetype. See Filetypes for a list of Perforce filetypes.
If you chose Run->Change filetypes... from the Path Browser, the form allows you to select the files on which the reopen command will operate. See Restricting Files Used in Commands for more information.
Click the "Reopen" button in the "Change Filetype" form after you have selected files and entered a new filetype. Note that this command only affects files that are already opened in your workspace.