Hi,
We’ve covered so many times already the topic of updates on this forum. Please search and read them before posting another topic about it. Also, to assist you further with specific issues, please specify your installation type, configuration, logs, etc
To start, I’ve written a post here - Upgrade Gitlab-ce from 11.11.5 to 15.1.6 with all the tips and tricks, migration paths and links to official GitLab documentation.
To be more specific with your use case - according to docs, you should be able to upgrade to 15.11.x without issues.
Hope this helps!