If you want to remove the PayPal Express Checkout from your Shopify storefront then you would first want to disable that payment gateway from within your Shopify Admin. Following the below instructions would disable the payment gateway so your customers could no longer put through orders with PayPal.
To disable PayPal:
To add a different payment gateway to your checkout, you would want to find a payment gateway that is available to you. You can check out this list which will show you which payment gateways are available to you by the location you're selling from. Next, you would want to enable that gateway from within your Shopify Admin. You can add the payment gateway of your choice by simply going into your Shopify Admin> Settings> Payments section and enabling the gateway from there.
I would recommend reading through our help guide, here, which goes over how to choose a payment gateway that will suit your business. If you're wanting to use a third party payment gateway to accept credit cards then I would suggest walking through our step by step guide, here, for some assistance.
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!