I have a matrix named "spec" (see below), it is a 6x3 matrix, and each element of spec is a list. For example, spec[1,"wavenumber"] is a list, and it contains 1876 numeric numbers and NAs. I want to replace the NAs to zero, but don't know how to change it, the difficulty may be all the elements are of the class list, so it is hard to change. 

Thank you for your help! 

matrix spec:

     wavenumber   prescan      postscan    
H001 Numeric,1876 Numeric,1876 Numeric,1876
H002 Numeric,1876 Numeric,1876 Numeric,1876
H003 Numeric,1876 Numeric,1876 Numeric,1876
H004 Numeric,1876 Numeric,1876 Numeric,1876
H005 Numeric,1876 Numeric,1876 Numeric,1876
H006 Numeric,1876 Numeric,1876 Numeric,1876
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