@Emran Hossain Thanks for reaching out.
If your entity is queue and want to read the messages from dead letter queue then you can leverage the When a message is received in a queue (auto-complete) which will read and complete all the message once the message is landed in dead letter queue depending on what configuration you have specified in the 'How often do you want to check for items?' property. Make sure you have select the Queue type 'DeadLetter' as in the below configuration.
Similarly if you have topic subscription as the entity then you need to leverage the as the When a message is received in a topic subscription (auto-complete) trigger.
In case if you want to manually run your logic and not when the message land in dead letter queue then you can leverage the Get messages from a queue (peek-lock) and then call the complete action. Similarly you can do the same for topic subscription. For more details on the different available action you can refer to this document.
To get started with logic app service bus trigger/action you can refer to this document.
Feel free to get back to me if you need any assistance.