If you forget your passcode, you can contact your Pleo admin who will be able reset it for you. Here is a guide on how to do it.
If you already know your passcode, and would like to change it, you can do so on the Pleo app as well as the webpage.
Changing your passcode in the app
Tap on the bottom right icon
Under Security, select Passcode, followed by Change passcode to replace your current passcode with a new passcode
Changing your passcode on the webpage
Click on your name in the bottom right
Select Settings - My Account
On the bottom right corner, select change to replace your current passcode with a new passcode