Double-click a custom column, the normal column editing paradigm.
If it is a column built from other columns (template column) it will pop up the template editing dialog. Which has a live preview of the results for the current book record.
It doesn't require a restart.
No need to go digging in the Preferences for the column creation dialog.
Which does require a restart, since it is much more powerful and is meant to modify the column itself, not just the default template.