You will need to use the Operating system (Mac Finder / Windows File Explorer) to copy and paste the Presets folder, until Adobe restore the 'Export' function to Presets.
(Method detail may be different if you store presets with the catalog!)
1. Open the Preferences dialog, Presets Tab, and click to show the Presets folder.

2. The {Lightroom} folder will be highlighted, Scroll up the folders in the window to the {CameraRaw} folder and open this folder-

3. The {Settings} folder in {CameraRaw} contains the XMP Preset files. Copy this {Settings} folder to place in your Dropbox folder.

Hint: In Windows, I have placed the {Settings} folder in my Quick Access panel of File Explorer for easy access to copy. But **Warning** do NOT rename the folder in the Quick Access panel as this will rename the folder making it and all your Presets disappear from Lightroom!