Okay, I think you are asking about a custom field on the APDL product and not Logbook Pro? I'm not sure what 'Update' button you're talking about. Is this on your PC or PDA?
If in the Spreadsheet Style log as an example, and you make a change such as adding a value to your custom field, click your mouse in the row above it and it will automatically save, as well as when the program/window closes.
For issues with your PDA and APDL such as sync'ing data to Logbook Pro, please contact Auman Software for assistance. They handle all issues related to porting data from APDL to Logbook Pro.