Hello @arham !
Yes you can change the Add to cart button to Buy now it will lead to check out. Follow these steps:
From your Shopify Admin, in the ONLINE STORE channel, click THEMES.
Select the [ ... ] button on the top-right corner and select EDIT LANGUAGE.
Click on the PRODUCTS tab, and change the text in the "Add to cart" field to "Buy now"
Then click SAVE, and you're done!
Hope this helps!