Peter Leow is right, however if you want to do it row by row you will need to add a WHERE statement to the UPDATE command to only update the record you just read.
You can also use the SqlConnection .Clone to copy the connection settings rather than repeat your connection string, you still have to open and close the clone as it only copies the settings from the source.
SqlConnection myconnection1 = myconnection.Clone()
You still have the myconnection.close inside the using with the .open outside the using, one day you will be trying to debug a piece of code wondering why it fails when everything looks right, keeping matching lines of code at the same level helps prevent future bugs.