Try

UPDATE table.Statements

SET field03 = REPLACE(LTRIM(REPLACE(field03, '0', ' ')), ' ', '0')

WHERE field03 like '0%'

This will only try to update where field03 starts with a 0. An index on this would help

However if you only have numbers in the string, I would probably go for a statement more like

UPDATE table.Statements

SET field03 = CAST(CAST(field03 as bigint) as varchar(20))

WHERE field03 like '0%'

Otherwise you could do

UPDATE table.Statements

SET field03 = SUBSTRING(field03,PATINDEX('%[^0]%',field03),20)

WHERE field03 like '0%'