Where to use before update and after update in Salesforce?

Hi Suraj

You can find the info on this Apex documentation.

For Before update

Triggerevent -Event Can change fields usingtrigger.new

Allowed.

Triggerevent– Can update original object using an update DML operation

Not allowed. A runtime error is thrown.

Triggerevent-Can delete original object using a delete DML operation

Not allowed. A runtime error is thrown.

For After Update

Triggerevent -Event Can change fields usingtrigger.new

Not allowed. A runtime error is thrown, astrigger.new is already saved.

Triggerevent– Can update original object using an update DML operation

Allowed. Even though a bad script could cause an infinite recursion doing this incorrectly, the error would be found by the governor limits.

Triggerevent-Can delete original object using a delete DML operation

Allowed. The updates are saved before the object is deleted, so if the object is undeleted, the updatesbecome visible.

Hope it may help you