I would like to convert high frequency (such as monthly) time series
data to lower frequency (such as quarterly) data.

One way is illustrated by the example in the CSV file below.

If that doesn't suit your needs, more details about your requirements might
help.

------------- cut here ---------------
,,Quarter
Jan,57,=INT((ROW()+1)/3)
Feb,232,=INT((ROW()+1)/3)
Mar,76,=INT((ROW()+1)/3)
Apr,90,=INT((ROW()+1)/3)
May,32,=INT((ROW()+1)/3)
Jun,6,=INT((ROW()+1)/3)
Jul,98,=INT((ROW()+1)/3)
Aug,34,=INT((ROW()+1)/3)
Sep,21,=INT((ROW()+1)/3)
Oct,56,=INT((ROW()+1)/3)
Nov,89,=INT((ROW()+1)/3)
Dec,90,=INT((ROW()+1)/3)


Quarter,Sum,
1,"=SUMIF(C2:C13,A17,B2:B13)"
2,"=SUMIF(C2:C13,A18,B2:B13)"
3,"=SUMIF(C2:C13,A19,B2:B13)"
4,"=SUMIF(C2:C13,A20,B2:B13)"