Currently, my Jotform allows for my clients to pay via PayPal. Can I change this to also allow them to pay via a credit card?

You coined the term "also" so I'll assume you want to give your clients an option to pay either with PAYPAL or with CREDIT CARD.

You can approach this in 2 ways:

1. If you have a PayPal Business Account, you can use PayPal Pro as your payment field instead of the standard PayPal payment field. This should allow your users to pick, right on the form, whether they want to pay through PayPal or credit card.

Related guides:

How-to-Integrate-Form-with-Paypal-Pro

Difference-Between-PayPal-and-PayPal-Pro

2. If you don't have a PayPal Business Account though, you can still achieve this using a separate payment processor that allows credit card payments. You have plenty to choose form like Stripe, Authorize.net and many more.

The only downside is that you need 3 forms to make this work. This was discussed in great detail, with step by step instructions, on the article linked below.

Complete guide: How-to-Create-a-Basic-Multiple-Payment-form