using odoo studio I created a credit limit then set a default value,
the problem is: I'm trying to set credit limit for all existing customers using a default value,
is that possible and how?
Yes. It's possible.
Go to the List view of the Customers
Use Studio to add the Credit Limit field to the List view
Exit Studio, select all the Customers and edit one of the Credit Limit fields to input the default value. Odoo will ask you whether the value should be applied to all the selected rows.
Go into Studio again and remove the Credit Limit field from the view if you do not need it to be displayed.