When you want to edit the value of the field on the layout in the 'Edit Preview' mode, the edited value does not reflect the change.
Reproducing the issue
1. Open e.g. Sales - A/R → Sales Order and open any existing document.
2. Click on the "Preview..." icon.
3. In the form of the preview, click on the "Edit Preview" icon in the toolbar.
4. Double click on e.g. the field of quantity, or any other, where the TextFormat is "Number: n2" (number with 2 decimals).
5. Change the value to some other value and click on OK.
The value of the edited field does not change.
The value reflects the change.
As a workaround, for fields where you need to modify the value in the preview, change the field type in TextFormat from Number: n2 to General on the layout in the Properties pane, or if this modification in the preview is required rarely, you can change the type of the field also in the preview directly.