Give a role permission to edit existing records but restrict it from creating a new

Navigate to Setup > Customization > Workflow > New.
Enter Restrict Create Custom Record in the Name field.
Select the Custom Record Type in the Record Type field.
Check the On Create check box.
Set Trigger Type to Before Record Load.
Set the Event Type to Create.
Click the button next to the Condition field.
Select User Role under the Field column.
Set the Compare Type to any of.
Click the Select Multiple button in the Selection column.
Select the roles that should be restricted from creating new records.
Click Done.
Click Save.
Check the Execute as Admin check box in the Basic Information section of the workflow summary.
Set the Release Status to Released.
Check the Enable Logging check box.

Save.
Click the New State button under the Diagram section
Enter a Name for the State.
Save.
Click the State button under the Diagram section.
Click the New Action button on Action tab.
Click Return User Error.
Set Trigger On to Before Record Load.
Enter an error message that should be displayed if a user attempts to create a new record in the Text field.
Save