There are two ways to read that question.
If you mean you want to convert an issue from a standard type to a sub-task, then go to the issue and look in the actions menu - there is a "convert to sub-task" option in there which will ask you for the new type and what the parent should be (note that you must have at least one sub-task type available in the project. You can also convert sub-tasks to standard types in the same way)
If you mean you want to convert the whole issue type, then there's no quick way to do it. You have to add the new type, convert all the existing issues to the new type, then delete the old one.
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