Anytime you need to reassign a Task, click the Related To Field

If blank, the system will show the Accounts...

If you click on the Account Icon, additional "Objects" will appear. Find the Opportunities or any other record Object Type you are searching for, and reassign it to the desired record.

Salesforce

It explains how to navigate the Related To field, find and reassign opportunities or other record Object Types, and utilize the Related To field effectively. If you want to efficiently manage and reassign tasks in Salesforce, this guide is a must-read.

How to reassign Related Objects