To Edit an existing field, click on the field name or the blue edit button located under the actions column.  You will then be directed to the Create/ Edit Fields page.  This will start the field create/edit wizard, and you will be directed through the series of steps necessary to Create/ Edit fields in the database. 
Step 1: Type
Start by filling in the Field name and Field Type. Once you are done editing this page, click the green next button. 
Note: if you are editing a field, you can only change the field type to compatible types. This is a restriction in Act! as to prevent data loss from converting incompatible types.
Step2: Properties
In this step, you can check or uncheck the properties you wish the field to have. When you are done editing, click the green next button. 
Step3: Drop Down
If you did not select to link the field to a dropdown in step 2, this step will be skipped. 
If your field is linked to a Drop Down, you will be allowed to select what types of properties you would like the drop down field to have.  When you are done, click the green next button. 
Step4: Defaults
In this step, you can set default values for the field.  If you wish you may leave these blank. 
Also, you may set the default field length.  It is automatically set to a default of 50 unless otherwise specified. When you are done, click the green next button. 
Step 5: Finish
If you are finished creating/editing the field, click the green finish button. 
Remember, once you will click "Finish" button changes will be saved and applied to ACT! Database. The task will be queued.
TIP! Before you create or edit the field, If you do not wish to lock the Act! database at this time, you can pause the database and the tasked will be  queued until the database is un-paused.  For more information on this, see Status Widget under the Database Dashboard section.
Next : Field security