After submission of a webform block how to replace that webform block with thank you message in the same page?
If you have the webform set as a block, you can go to admin > structure > block. Look for your webform block. It should be named something like Webform: 'name of webform node'. Click on the configure link to the right. When the edit form for the block loads, look for the section Confirmation message. Select the option Display the confirmation page in the block on the same page (no redirect). Now when you submit the form from the block, it will redirect back to it and show the confirmation message on reload.

You may need to fill out a message in the webform settings. I have attached an image of the edit block page for you to compare.