Optimize SELECT UNION

There is a stored procedure in one of DBs (and the DB size is about 70 GB) which is as below.

SELECT ...
FROM {Inner Join of 10 tables and views}
GROUP BY ...
UNION
SELECT ...
FROM {Inner joins of 10 tables and views}
GROUP BY ...
UNION
SELECT ...
FROM {Inner joins of 10 tables and views}
GROUP BY ...
UNION
SELECT ...
FROM {Inner joins of 10 tables and views}
GROUP BY ...
I don't know the DB yet and have recently inherited it. Is there a better way to code the SP in order to optimize and increase its performance?
thanks,

If it's basically the same query statement each time, but with a different WHERE clause, then I'd check if procedure parameters are also part of the criteria, like this:
CREATE PROC MyProc (@p1 BIT, @p2 BIT)
AS
SELECT col1, col2 FROM MyTable WHERE @p1 = 1 AND col1 = 'x'
UNION
SELECT col1, col2 FROM MyTable WHERE @p2 = 1 AND col1 = 'y'
For this kind of conditional filtering a better option might be to use dynamic SQL in your procedure, by calling sp_ExecuteSQL with proper parameters.