I am not sure if management studio is limited in SQL Express, but usually you can just Right-Click on the stored proc name and select "Script Stored Procedure As" then select "Create To" finally select an appropriate option and save the query in a good place.

If by chance the management studio for SQL Express does not have this option, you can run this command and then save the query results somewhere.

exec sp_helptext 'ProcedureName'