Where to Customize Modification Confirmation Email?
You can modify the confirmation email sent to users after upgrading/downgrading levels under OptimizeMember >> Stripe Options >> Modification Confirmation Email.
Please Note that the email configuration even if it's under the stripe options is universally applied to all Payment Gateway integrations. Thus, it should work on paypal integration as well.
In case you hadn't enabled stripe option yet, please go to OptimizeMember >> Other Gateways >> Tick on Stripe payment option and save settings from there.