The Developer must do it in Temp server After successful We should have time to do more and more testing to insure the Upgrade is OK. Then He should The following:
1-Write Document what he did to do upgrade on steps.
2-Do it on The our Customer Server -- After planing the time with customer.
3-Ensure all the Features are working Fine.
What we did on ERPNEXT We did Invoices and numbering and we change 2 line of code in one or files and it is noted.