[Koha] Upgrade koha 3.8

Dear all, I want to upgrade my koha 3.8 to 3.12. kindly help me on this. I also need to upgrade the same database to 3.12. I have download the koha using git. I am using git checkout -b brachname origin/3.12.x to upgrade the koha but its not working got errors. Please sends some steps to upgrade the koha from 3.8 to 3.12 with current database. regards, Rocky Singh