Like Kagithala mentioned it's not possible today. You could vote for this Atlassian ticket to request this feature, but it seems that they're not planning on working on this feature at this time:
More details can be found in this other thread: https://community.atlassian.com/t5/Jira-Software-questions/How-can-I-change-default-issue-type-for-a-next-gen-project/qaq-p/1148127
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