Postgres 13

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create table t_test_result (test_json jsonb);
insert into t_test_result values 
	('{"test_name": "Full", "parameter1": "4.219", "parameter2": 4.4137}'),
	('{"test_name": "Full", "parameter1": "3.758", "parameter2": 4.159}')
2 rows affected

update t_test_result
set test_json = jsonb_set(
    test_json, 
    '{parameter1}', 
    to_jsonb((test_json ->> 'parameter1')::numeric)
)
where test_json ? 'parameter1'
2 rows affected

select * from t_test_result t


test_json


{"test_name": "Full", "parameter1": 4.219, "parameter2": 4.4137}

{"test_name": "Full", "parameter1": 3.758, "parameter2": 4.159}