How to obtain the Channel_ID from the Short_Channel_ID? Is there a way to retrieve the Channel_ID knowing the Short_Channel_ID without using any lightning network explorer? 

How is the Short_Channel_ID calculated? Is this operation reversible?
The short_channel_id points to the funding transaction of the payment channel by the format blockheightxtransaction_indexxoutput where the x is used to seperate the 3 values and the three values are depicted in decimal. This is at least the BOLT 1.1 agreed format for the short channel id. Previously the shortchannel id might have been encoded and depicted differently depending on the implementation. For example c-lightning used a colon instead of an x to separate these fields. 

According to BOLT02 the channel_id is created as follows: 


  This message introduces the channel_id to identify the channel. It's derived from the funding transaction by combining thefunding_txidand thefunding_output_index, using big-endian exclusive-OR (i.e.funding_output_index` alters the last 2 bytes).


as the funding_txid is a hash of its fields this has no relation with the short channel id. So without looking at the blockchain data I believe it is impossible transfer the short_channel_id to the channel_id or back.