Get first or last value in a group

For most SQL dialects, the easiest way to get the first (or last) value within groups is probably to add a row number, and then filter on 1 in the next step:

WITH ranked AS (
		SELECT
				user_id,
				balance,
				ROW_NUMBER() OVER (
						PARTITION BY user_id 
						ORDER BY balance_updated_at
				) AS rnk
		FROM users_balance
)

SELECT
		user_id,
		balance
FROM ranked
WHERE rnk = 1

If you want to get the last value, just use ORDER BY balance_updated_at DESC