This depends on where you started the free trial.
Please see the relevant instructions below.
Apple App Store
Your subscriptions and payments are processed through your Apple ID account.
To view, change or cancel your subscriptions, please sign in to your Apple ID account. Instructions are available here:https://support.apple.com/HT202039
Google Play
Your subscriptions and payments are processed through your Google ID account.
To view, change or cancel your subscriptions, please sign in to your Google ID account. Instructions are available here:https://support.google.com/googleplay/answer/7018481