Mass update all cells in a column with a substring of it's original value

I need to trim the last seven characters off of all the cells in a column. This query returns the results that I need SELECT test_date, SUBSTRING(test_date FROM 1 FOR CHAR_LENGTH(test_date) - 7) as test from results; I now need to take the computed result from the substring and replace the original values. Here's what the data looks like, if it helps.