Help on HP Laserjet 4 printers

I have quite a few HP LaserJet 4 plus printers. I’m having problems with the way the paper is taken from the trays.
When you select ‘upper tray’ on the print menu, the printer feeds from the lower tray and when you select the ‘lower tray’ it feed from the upper one. My users are getting annoyed and any help would be appreciated.

Many Thanks

Neil Hughes