See -> https://www.linkedin.com/help/linkedin/answer/a563180/cancel-linkedin-premium-subscription-purchased-via-itunes?lang=en
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Due to Apple's privacy policies, you can’t cancel a LinkedIn Premium subscription purchased via iTunes directly on LinkedIn. If you’ve bought your Premium subscription via iTunes or the App Store, you’ll be able to begin the cancellation process on the LinkedIn desktop or the LinkedIn mobile app. You’ll then be redirected to the iTunes subscription management settings page to complete the process.
See also -> https://www.linkedin.com/help/linkedin/answer/a565269?lang=en
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We will send an email to the primary email address on your LinkedIn account seven days prior to the end of your free trial for any subscriptions purchased via desktop.
So not sure if you can get refund, if you forgot to cancel the trial version.
Contact LinkedIn support.