Change any keyword(variable) into all stored procedures in two steps.
select
replace(replace(definition,'create','alter'),'x','y')
from sys.sql_modules
where definition
like'% procedure %'
or definition
like'% proc %'
It will show a list of definition of all stored procedures.
Just Copy all rows by select all option and paste on query editor and click on
run, it will alter all your procedures in one shot.