I have few attributes in an item which are not PK/SK/GSI but null. So when I store then in Dynamo DB it shows them as Null with Type as Null. Is there any way where I can change the type from Null to String/Boolean ?
You can't change a data type without updating each item you want to change one-by-one.
DynamoDB has a Null data type, but it's best to drop null values and not store them at all.
In short, to overcome your issue you'll have to make a change on how you write items.