There are few changes needed for upgrading from v2 to v3. but the bulk of changes can be done by leveraging the power of find and replace on your editor.
Change ^2.xx (xx can vary) to ^3.0 in your composer.json
Runcomposer update. Of course, your app must meet the minimum requirements as well.
Change PayPalClient namespace from PayPal\Checkout\Http\PayPalClient to PayPal\Http\PayPalClient.
Change environments namespace:
PayPal\Checkout\Environment\Environment to PayPal\Http\Environment\Environment.
PayPal\Checkout\Environment\ProductionEnvironment to PayPal\Http\Environment\ProductionEnvironment.
PayPal\Checkout\Environment\SandboxEnvironment to PayPal\Http\Environment\SandboxEnvironment.
Change requests namespace:
PayPal\Checkout\Http\OrderCreateRequest to PayPal\Checkout\Requests\OrderCreateRequest.
PayPal\Checkout\Http\OrderCaptureRequest to PayPal\Checkout\Requests\OrderCaptureRequest.
PayPal\Checkout\Http\OrderAuthorizeRequest to PayPal\Checkout\Requests\OrderAuthorizeRequest.
PayPal\Checkout\Http\OrderShowRequest to PayPal\Checkout\Requests\OrderShowRequest.
PayPal\Checkout\Http\PaypalRequest to PayPal\Http\PaypalRequest
There are no special requirements for upgrading from v1 to v2, other than changing ^1.xx (xx can vary) to ^2.0 in your composer. json and running composer update. Of course, your app must meet the minimum requirements as well.