If you want to set the environment, then - set NLS_LANG=american_america . So you change the environment only for client programs.
I understand you need to change at the level of DB. Then you need to set NLS_LANGUAGE and NLS_TERRITORY separately in the init.ora file.
NLS_LANG is not used in init.ora.
UPD The above said concerns the language and territory. The encoding is set to NLS_CHARACTERSET, i.e. v 3rd component NLS_LANG (languge_territory.characterset).
The encoding can only be changed if the new encoding is a strict superset of the old encoding, i.e. WE8ISO8859P5 cannot be changed to AL32UTF8, and US7ASCII is always possible.
If lucky, then alter database character set AL32UTF8; .
If not, data migration and, in the worst case, programs that do not quite understand the new encoding. Read more here or in general in Russian here .