Customising subscription message page

Hi, the plugin works great, thanks for your help.

I would like to customise this message that new subscribers see on the website after clicking in their email:

Congratulations! You’ve successfully subscribed!
Back to homepage ?

I’ll probably only change the text at this stage but later I may want to add other links.

Is this message set within WordPress itself or a file within the plugin? I’ve searched everywhere but I’m guessing you’ll know straightaway.

https://www.ads-software.com/plugins/simple-subscribe/