I need help updating the theme for my website so PayPal Express, Shop Pay, Apple Pay will show up

[JeriHuff]

One of my customers notified me that the option to check out using PayPal Express is no longer available on my website.  After reaching out to Shopify support I learned that the theme used for my website is out of date and needs to be updated.  Since my website wasn't built using one of Shopify's themes they can't help me with the update.  Can someone please help me with this?  Is there a lot that goes into updating a theme?

Thank you!