Changing database collation for tempdb is little different than changing collation for any other system or user database. Here, you cannot change tempdb collation with ALTER DATABASE command.
Instead, you have to change the collation of model database to the required collation setting that you want for tempdb. After that restart SQL Server services and you will find that tempdb collation has changed.
Note:- Remember whenever SQL Server services are restarted, tempdb is recreated using model database and it will inherit all the properties of model database.