I’ve had the same question recently and did a little bit of investigating as most of these posts go unsolved.
Lo’ and behold I found the answer.
First you go into your online store and choose “edit code” in the actions menu. Then find the “cart-template.liquid” file and locate the 268-269th row.
Implement your paypal button code inbetween the 268th and 269th row as shown
And then you shoud comment out the code that functioned as your normal check out button as shown below.
And this is how the code looks implemented in your store
