Move your thank you message to a new block. In the block options for that block, you can change the Next/Back button text.

This risks the user not clicking the "Exit Survey" button and response not being recorded in a timely fashion. You can help fix this be recording partial responses but that might not be ideal in your situation...

What is the desired functionality of the Exit Survey button? Do you want it to close the window (problematic), direct the user to a new URL, or something else?

Hi Radam
I would like the exit button to either close the window or direct the user to a new URL. Anything which would indicate that the survey is over. Closing the window would be ideal unless it's too problematic.

ahmedA
How can I change the next button for only 1 block?
I actually have 3 different survey endings based on the answers.

Open your survey and go to block where you wish to change the button name
click "Block Option" and select "Next/Previous Button Text" a popup screen will
Change the button name.

https://www.qualtrics.com/community/discussion/comment/32402#Comment_32402Most browsers security won't allow JS to close the browser unless it was opened using JS. Most user know a survey is over when you have the "thank you for taking this survey" page.

https://www.qualtrics.com/community/discussion/comment/32403#Comment_32403If you have three different survey endings, and you want to create the exit buttons for each of them. You'll can either put them in one block and use display logic to show only the ending you want. Alternatively, you can put them in separate blocks and use branching in survey flow.
However, I do agree with Radam it is advisable not to have an exit survey button at the end, since there is a high probability that people won't click it. It is better to show customized end of survey messages, which you can do via the library, and display them accordingly using branch logic.