What do you want to "modify"? the sliders in the Develop panels after applying a Preset, or the settings contained in the Preset ?
To modify a 'User' Preset settings, I would (after the image [Reset] ), apply the Preset, alter the sliders, [Right-Click] the Preset and [Update with Current Settings], or- create a new preset from the current Develop settings.
The Adobe default Presets cannot be modified- you can only create a new preset after applying.
Possibly, Purchased Preset files may be 'Read Only' and cannot be changed by Lr.