Code:
# cat file
16458,99,001,"RIMOUSKI, QC",418,"N",7,EST,EDT,902
16458,99,002,"CHANDLER, QC",418,"N, C",5,MST,MDT,616
16458,99,003,"NEWRICHMND QC",418,"B, C",7,EST,EDT,702
# sed 's/,/ /4' file
16458,99,001,"RIMOUSKI QC",418,"N",7,EST,EDT,902
16458,99,002,"CHANDLER QC",418,"N, C",5,MST,MDT,616
16458,99,003,"NEWRICHMND QC" 418,"B, C",7,EST,EDT,702
# awk 'BEGIN{FS=OFS="\""}{sub(",",x,$2)}1' file
16458,99,001,"RIMOUSKI QC",418,"N",7,EST,EDT,902
16458,99,002,"CHANDLER QC",418,"N, C",5,MST,MDT,616
16458,99,003,"NEWRICHMND QC",418,"B, C",7,EST,EDT,702
# sed 's/, / /g' file
16458,99,001,"RIMOUSKI QC",418,"N",7,EST,EDT,902
16458,99,002,"CHANDLER QC",418,"N C",5,MST,MDT,616
16458,99,003,"NEWRICHMND QC",418,"B C",7,EST,EDT,702
# sed 's/"\([^"]*\),\([^"]*\)"/\1\2/g' file
16458,99,001,RIMOUSKI QC,418,"N",7,EST,EDT,902
16458,99,002,CHANDLER QC,418,N C,5,MST,MDT,616
16458,99,003,"NEWRICHMND QC,418B, C",7,EST,EDT,702