T-code: ZTCURMNT is Posting Currency Rate.
I'm showing an example of AUS.
1) Double click AUS.
2) Input currecy rate & press "F8" key.
Rate: EUR/AUD
Date: 02.01.2021
Exchange Rate: 1.58535
3) Status is changed as Green & Save it.
After saving if you want to revise the rate, overwrite and save it.