Can I submit an iOS App update to the App Store but hold the release of the update for a later date? I have an app that I have to update to work with the new backend.

The new backend will be deployed some time in the next X days. 

Is it possible to submit the update for review, but not have it released immediately after it gets approved, so that I can release the new version of the app after I swap the backend with the new one?
Is it possible to submit the update for review, but not have it released immediately after it gets approved


Yes. It's possible to do so.

When submitting your app for review on App Store Connect, simply select the option to manually release the app instead of automatically release. This way, once the app is approved by Apple, it is kept at hold for release on the App Store until approved by the developer. You are also notified by email by once the app update is approved.

Refer to the following linked App Store Connect Help documentation pages:


Manually release a version



  If you choose to manually release a version when you submit your app for review, you can release the version after it’s approved and the status changes to Pending Developer Release. If you have an app in the Pending Developer Release state for longer than 30 days, you’ll receive an email reminder from Apple.



Submit your app for review



  
  In the Version Release section, select a release option:
  
  
  Release the app yourself: Select “Manually release this version.” (Later, manually release the version.)