I have UWin version of Unix for Destop

I have files with extension *.abc. I want to rename it to *.csv.
Some of the filenames have spaces.
I also to rename the files with extension *.abc in the corresponding subfolders
ex
==
abc def.abc
ghi jkl.abc
mnopqr.abc
uvwxyz.abc

rename as
========
abc def.csv
ghi jkl.csv
mnopqr.csv
uvwxyz.csv