After you've downloaded a project and selected a different location this one is seen as the default. Next project you'll download will automatically be placed here.
When you open the modeler, select open. You can specify the directory, this will be the default, from now on.
I don't know how far you are with your app, if you just started, try using 7.16. Or migrate to 7.17.x as soon as it releases.
Based on https://docs.mendix.com/releasenotes/desktop-modeler/7.17
" We have decided to retract version 7.17 due to an issue with the Session manager in clustered environments and an issue that breaks the styling of your app once deployed to the Cloud. This will be fixed in 7.17.1 "
Is this ‘new default location’ stored for all projects and modeler versions? So if you start a new project or install a new modeler version, will the location still point to your location of choice?