I dont think we can update the a369i or Lenovo has any plans of updating the OS. That is probably the reason the button is missing.
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I do not know the reason why play store shows many apps as incompatible. Many common apps including those requiring low end specs also show the phone as incompatible. But these apps work fine once installed though installing itself is tricky.
If you are very sure the phone has sufficient hardware requirements, you may install the apps using apk files.
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I have installed few apps that I needed using apk files and using them without any issues.
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