Go to Options
Click on System Maintenance
Choose System Settings
Select GENERAL System Settings
Click on User Defined Field Titles
Select from the list the specific area in the system that the field you wish to change exists (e.g. Purchase Order Header)
Type Ctrl+M to go into modify mode
Tick the 'Override' tick box
In the 'Title to show' area, type in your chosen text (e.g. Proof of Purchase)
Now when you go into the area you have chosen to make the amendment, you shall see the updated field title there.
