Can I upgrade the mini ipad 2 OS from 9.3.5 to 12?

Yes, if it really is a "2."

If it has been that long since you upgraded it, you may need to connect it to a computer running iTunes instead of updating "over the air." Update your iPhone, iPad, or iPod touch - Apple Support

If iTunes does not show an update available, maybe you have an iPad mini 1, which is hardware-constrained to running no higher iOS version than 9.3.6 (9.3.6 on some cellular models). For help in getting a positive ID for your iPad model, please see this Apple article:

Identify your iPad model - Apple Support

The iPad Mini 2 is compatible with iOS 12 directly, so the update should show up in Settings->general->Software update.

If its not, you likely have an iPad Mini 1 and it cannot be updated beyond iOS 9.3.5

See here to identify your actual iPad Mini model: Identify your iPad model - Apple Support

If it is actually an iPad Mini 2, then you can try to update through iTunes on a computer instead: Update your iPhone, iPad, or iPod touch - Apple Support

The 1st gen iPad mini cannot be upgraded past 9.3.5. (9.3.6 for the cellular version).

You are better off buying a new iPad, remember the iPad mini 1st gen was released in 2012. The regular iPad 7th gen sells for 329$. You can also look at refurbished models on Apple's website (US only), they are a bit cheaper.