SQL query to replace attribute value with another attribute value- postgre
Easy with a CTE:

with updata as (
select user_id,attribute_value 
from eavtable
where attribute_key='newEmail'
)
UPDATE eavtable
SET attribute_value = updata.attribute_value
FROM updata 
WHERE eavtable.user_id = updata.user_id 
and eavtable.attribute_key = 'Email';


Then a delete:

delete from eavtable where attribute_key = 'newEmail';


DB Fiddle Link.

Might also be able to do the whole thing in a single merge.