How to modify a stored procedure using SQLCMD for MS SQL?
Assuming that you have the permission to do so then there should be no reason why you could not. The following example assumes that you are using Windows Authentication:

sqlcmd -E -S computer\instance -d database

alter procedure testtable_pricesmaller 
@pricelimit money
as
select * from testtable where price = @pricelimit;
go    


Alternatively if you have an existing script file that has the command you want to run you could use the -i option to run that file:

sqlcmd -E -S computer\instance -d database -i file.sql


Here is some more information on the sqlcmd utility that you should find helpful:

http://msdn.microsoft.com/en-us/library/ms180944.aspx

I hope this helps you.