how to copy the value of an old field to a new one.

[larisa]

Hello.
To optimize the work, you need to change the fields for bugs in Jira.
(In particular, the 'Description' field to change by the customer field). I need to create a new customer field and hide the old one, but then the data from the field of previously introduced bugs is lost.
A problem arose about how to copy the value of an old field to a new one.
Please tell me how to do this.