Hi @Daipayan_Mandal,
You can change the data type of a field by using the data type selectors on the right of the field in the Atlas Cloud Cluster as well as MongoDB Compass.
For more info on data-type read here
If you want to update it using Mongo shell or any specific drivers of MongoDB then you can refer to the $convert operator.
Hope it helps !!
All the Best
Kushagra