[R] substituting level for NA in factor column

[Kurt_Helf at nps.gov]

Greetings
     I have a bunch of NAs in a column of categorical variables designating
the size classes (e.g., smallest to largest: 1,2,3,4) of cave crickets.
I'd like to substitute "U" (for unknown) for the NAs.  Can anyone give me
an idea how to do this?  Thanks in advance.
Cheers
Kurt

***************************************************************
Kurt Lewis Helf, Ph.D.
Ecologist
EEO Counselor
National Park Service
Cumberland Piedmont Network
P.O. Box 8
Mammoth Cave, KY 42259
Ph: 270-758-2163
Lab: 270-758-2151
Fax: 270-758-2609
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