Converting varchar to decimal with truncate

I have a column called latitude which is currently varchar(20) I want to convert it to DECIMAL(9,6) However the data stored inside the column is greater than 6 decimal points i.e. 48.123456891123456 the table in question has over 50 billion rows and is a 24/7 database with no downtime and uses partitioning by month (SQL server 2017 enterprise) How would I achieve the conversion as its too big for 6 decimal points. I was thinking of creating a copy of the column and renaming when converted, however i'm not sure how i would achieve the truncate to 6 decimal points