Tap on the 3 horizontal stacked lines at the top right corner. You will see multiple options like Your orders, Your account settings, Instacart express, etc.
Tap Your account settings.
Tap Account info (first option). On this account info page, you will be able to see your account details like email, your name, and phone number.
Tap on password at the bottom.
Now enter your Current password in the first column and then enter the new password and enter the new password again in the third column to confirm your new password.