I have been asked this on site a few times over the last few months and thought it worthwhile to post the answer for anyone looking for an answer.
Is it possible to change the order of the User Defined Fields (UDFs) in SAP Business One?
The system automatically assigns a numbering order to the UDFs as they are created. It is possible to change the order of the UDFs. You have to open the form that you wish to change the UDF order of and ensure that you can see the UDF window. You then go to Tools/ Customisation Tools/ Settings/ Order and reassign the numbering order here. Number 1 will be the top UDF displayed. Please note that it is worth mentioning that if you have over 100 UDFs this may take some timeif you want a completely different order than already displayed!
Another solution is to apply categories to the UDFs which you can select at the top of the UDF window on the form. This is a neat and simple solution if you are able to categorise your UDFs in a sensible manner.