Press 'Esc' to bring up the GUI
Swap the discs (physicaly)
Go to 'File' > 'Change Disc' > then choose between 'ISO' or 'CD-Rom' dependant on whether you are using ePSXe's internal ISO support or not.
Swap the discs (physicaly)
Go to 'File' > 'Change Disc' > then choose between 'ISO' or 'CD-Rom' dependant on whether you are using ePSXe's internal ISO support or not.