Update iPad mini 1st gen

What's the current version number on the device in Settings > General > Software Update? The last iOS version released for the iPad mini (1st generation) is iOS 9.3.5 (or 9.3.6 if it's a model with cellular), so if you have this, the device is already up to date. Well, it's outdated software, but it's the newest available for this particular device.

Yup,

The 1st gen iPad Mini cannot upgrade/update beyond iOS 9.3.5/9.3.6 for cellular data models.

Your iPad Mini is nearly 8-years old now!

These old 8 and 9-year old iPad models are too old, now, too underpowered and completely incompatible with any of the current and future versions of iOS/iPadOS.

Also, tap and read my link below for my additional info about your nearly 8-year old iPad model.

If you own OLD 8 and 9 Year Old iPad mo… - Apple Community

Sorry & Good Luck to You!