Welcome to Android Central! I moved your post to create a new thread, since you initially responded to a very old thread for the Galaxy S4 phone.
I assume you're talking about the M-PESA app by that carrier?
https://play.google.com/store/apps/details?id=com.safaricom.mpesa.lifestyle&hl=en
Go to
Settings>Apps, select the M-PESA app, then Notifications. Then tap the
words of the notification category you're interested in changing (not the toggle switch). In the subsequent menu, tap Sound, and choose your new notification tone there. (The exact menu selections might be a little different on Samsung -- I'm going by the Pixel 7 Pro menu, because I currently don't have my Galaxy A32 5G in front of me.)