That depends on what you want to do. To remove all leading and trailing spaces, you would do:

UPDATE tbl

SET col = rtrim(ltrim(col))

WHERE col <> rtrim(ltrim(col))

To collapse multiple sequences of spaces, you could do:

UPDATE tbl

SET col = replace(col, ' ', ' ')

WHERE col LIKE '% %'

The later UPDATE does not handle tabs, which you need to handle separately.