Understanding income statements

In case of change in accounting policy at the end of year 2020 , let’s say the value turns out to be 500 by the retrospective method and earlier it was 400 at end so this profit of 100 will appear in which P/L statement 2019-20 or 2020-21 ?

IFRS states " entity shall adjust …comparative amounts disclosed for each prior period presented as if the new accounting policy had always been applied." - full retrospective application

The change occurs in 2020 so prior years will be restated (from 2019 or earlier)
The 2021 beginning equity (usually retained earnings) will have a restated balance sheet. 2021 P&L is not affected and it will apply the new accounting principle as usual.

It depends on what you mean by an accounting policy.

If you change an accounting principle (e.g., from straight-line to accelerated depreciation), then, as JuanCruise mentions, the change has to be applied retrospectively.

If you change an accounting estimate (e.g., you change the salvage value), then the change is applied prospectively.