How do I receive a subscription refund?

First, see if the subscription shows here and cancel if necessary. LinkedIn offer subscriptions both through an in-app purchase that is managed by Apple and also directly with them on their website where you would need to cancel there. If it is managed by Apple, you will be able to see it here and can cancel.

View or cancel app subscriptions on iPhone - Apple Support

If it was managed by Apple, you can try and request a refund here:

Request a refund for apps or content that you bought from Apple - Apple Support

In most cases all transactions in the App Store are final, but at times Apple may offer a compassionate refund. It should never be expected though. If you started a free trial and forgot to cancel the trial at least 24 hours before the trial period ends, you will be charged for a subscription. Usually, forgetting to cancel a free trial has not been seen to be a valid reason for a refund. If you use Free Trials, I would recommend setting a Reminder so you do not forget to cancel.

I accidentally subscribed to LinkedIn premium subscription and have been charged P1,490 how do I receive a refund

See the post directly above yours. It has the link to request a refund.