How to change financial year in tally for 2020-2021?

1. Never split company data now

Never split data immediately to welcome the next financial year unless you finish recording all pending bills pertaining to the financial year 2019-20, do the necessary adjustments in entries, complete ledger reconciliation and internal auditing etc.
Open Tally.ERP 9 and change period (Alt + F2 -> to change period) to 01/04/2020 to 31/03/2021. Now ensure that the current period depicts as follows:

Simply change the date to 01/04/2020 and enter your 1^st transaction of the new financial year.
It is better to change the date in full format such as DD/MM/YYYY and continue your next financial year transactions.

Once you finish recording all pending entries pertaining to the financial year 2019-20, adjustment entries, ledger reconciliation, internal audit etc., do splitting Tally data to keep the 2019-20 data separately.
Important note: Ensure that you have taken backup in multiple places before you do splitting data.
Open Tally.ERP 9 data for the period from 01/04/2019 to 31/03/2021. Gateway of Tally.ERP9 -> Alt + F3 Company Info -> Split Company data -> Select Company and ensure that you split the data from 1^st April 2020 as follows:

Once this process is over, new financial year data to be recorded in the company from 1^st April 2020 to 31^st March 2021.
It is recommended not to make any changed in the old financial year data i.e. 2019-20.
If you are planning to make changes in the old financial year data (2019-20) then ensure that the closing balance of such ledger is updated in the opening balance of 2020-21.