updating a file with sed -help

I have a file of records all made up of single charactors that need to be updated using their row/column

(the dashes represent spaces as the html formatting of this text box when it posts removes the "nonessential" white space between the charactors)

file example

A  1  c
B  2  b
C  3  a

to change row 2, column 2 to "5" would look something like this correct?

sed -i -e '2s/?/5/2' FILENAME

two questions: What would I have to put in the "find" section to correctly replace a non-blank charactor with the "replace" section and how could I pass variables into each of the R/C/replace sections as

sed -i -e "$Rs/?/$U/$C" FILENAME 

doesnt work

what is 'blank'? [ ]
what is 'non-blank'? [^ ]

sed -i -e '2s/[^ ]/5/2' FILENAME

Thank you, thats exactly what I was looking for