Update field name without losing the value

I hope I understand the question correctly

UPDATE `wp_postmeta` SET `meta_value` = replace(meta_value, 'old_value', 'new_value') WHERE `meta_key` LIKE 'your_key'

EDIT 1 : i forgot to mention : BACKUP YOUR DATABASE BEFORE ANY TRIAL

EDIT 2 : following comment : To copy from one field to another (I got a bit confused with your naming and which value you want where , so I post the generic sentence.

UPDATE `TABLE` SET `FIELD2` = `FIELD1'