Android: How do you update the calculator?

[StephenG1CMZ]

I have the same Google account on two phones.
One has Version 8151, the 2nd phone doesn't.

As I understand it, Google Play is meant to synchronise files between devices - but the fact that it didn't install on the 2nd phone didn't bother me - all that synchronising would use up precious bandwidth.

But now there is a new version out, I'd like to install it on the 2nd phone (bigger screen) or both.

Looking at Google Play, it knows I have purchased the calculator, so I have the correct account details - but the only option I am seeing is to installing. I don't see any Update option.

Am I wrong in thinking that the 2nd phone should have installed 8151 automatically a year ago?
Am I wrong in thinking it can be updated? If so, how?
Or Do I need to repurchase it?
(Not a problem if I do, though I will wait until I get an SD card as storage is running g low).
I usually access data using a WiFi rather than 3G/4G if that makes a difference.