Go to Subscriptions → My Channels, choose a channel or group, then tap 🗑 Disconnect and confirm.

If you want to disconnect a channel or group that has subscribers, go to Subscriptions → My Channels, choose a channel or group, then choose 🍓 Edit Subscription Plans, choose an option, then tap Archive Plan and tap Block Renewals. Repeat this for all active subscription plans and wait till all subscriptions expire.