ls *.txt |awk '{print "mv "$1" " substr($1,1,(index($1,".txt")))"csv"}' |sh

ls *.txt |awk '{print "mv "$1" " substr($1,1,(index($1,".txt")))"csv"}' |sh

Command explanation:

ls *.txt lists files ending with txt

| awk '{print "mv "$1" " substr($1,1,(index($1,".txt")))"csv"} prints the command that allows to rename the previous files listed with txt to csv

| sh executes the previously printed command, thus effectively renaming the files

Note: if you want to list all files including hidden files (those starting with .) use ls -a *.txt instead

ls *.txt |awk '{print "mv "$1" " substr($1,1,(index($1,".txt")))"csv"}' |sh

ls *.txt |awk '{print "mv "$1" " substr($1,1,(index($1,".txt")))"csv"}' |sh