1. Go to Reports & Forms, Forms, select the type of form you would like to modify.
2. Select the form you need to modify, then click Customize. This will open the design view window.
3. Double-click the number field you need to modify. This will open the Column Data Options window.
4. Under Edit Fields, click on the # button in the Format column on the line you wish to modify.
5. From the Number Formatting window, add or remove the number of n's on the right of the decimal.  For example, if you need to increase the decimal places to 4, the number format should look like this:  --NN,NNN.nnnn.
6. After making changes, click OK, then OK again.  Finally click Save at the top of the form.  If you want to keep your original form and save the changes as a new form, change the name of the form, then click Save.

How to customize forms
How do I make a customized form my default form?