Task actual start date change task plan date

Hi,

I have created Project Plan set baseline, plan has task which plan to start 20 Jan 2021, but actually task has started from 04 Jan 2021.

when i update actual start date of task and select 04 Jan 2021 its also changed plan start date.

pls guide how to maintain actual start date, so plan date keep intact and only actual date change. so that we can track project.

Thx. 

iffi