Hi @AntonioFar
The short answer is that you can not go directly to 9.2 from 8.4. I would also highly recommend that you target an upgrade to 9.3.9 or 9.4 for supportability reasons.
Here is a link to 9.X upgrade section of help center, https://support.ptc.com/help/thingworx/platform/r9/en/index.html#page/ThingWorx/Help/Installation/Installation/upgrade_overview.html.
Here is a link to the 8.5.x Upgrade documentation which you could potentially do an in place upgrade. https://www.ptc.com/support/-/media/support/refdocs/ThingWorx_Platform/8,-d-,5/Upgrading_to_ThingWorx_8_5.pdf?sc_lang=en
As mentioned read through the release notes for the different versions. There have been several foundation upgrades that will most likely require your mashups to be reworked as well as improved functionality.
You should be managing your code in a source control repository. If you are not, now is a good time to start.
Thanks,
Travis