How to buy application previously downloaded from GetJar? I have Agenda Widget Plus downloaded from GetJar. Now I would like to pay for it, because I find it very useful and I would like to reword author of this app. On Play Store it is listed as already installed and I cannot pay for it.

Any idea how can I do it?
You will need to uninstall the app from your device and after that the Play store should allow you to purchase it.