It is done automatically if you assign the contents of one typed variable to another differently typed variable. You will get an exception if the conversion can't be done.
Dim S
Dim I As Integer
Dim N As Number
N = 1.234 ' contents are a Double value 1.234
S = N ' contents are a string "1.234"
I = S ' contents are rounded/truncated to an Int32 value of 1