AFAIK this is a limitation of the Installshield Express version
Need to purchase the full version if you want this functionality :(

>We built a setup.exe file with Installshield that came with VFP7.
>
>On the client PC, we ran the setup.exe file and the setup performed as expected.
>
>We made a change to the installation and re-created a setup.exe file, ran the setup.exe file again on the same client PC but Installshield would NOT continue with the installation because of a found previous install. The message told us that we had to go to the Control Panel and remove the previous installation before we could run the new setup.exe file.
>
>We did this and it worked, but why didn't Installshield just overwrite the previous files? Is there a setting I missed? I don't want our user to add/remove software when we send out updates.
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>Is there a setting in Installshield that will overwrite existing installs without any prompts?
>
>Thanks,
>
>Jerryt