Download it here:
This plugin increases the resolution of the 20m and 60m bands of a sentinel-2 image down to 10m/pixel. You can view it as a kind of pan-sharp operator, but which extracts details from the 4 existing 10m bands instead of a (non-existent) panchromatic band. Thanks to using these 4 bands, results are improved compared to single-band pansharp, including local consistency between neighbor pixels. See the above page for details.
The plugin works for both SNAP 5.0 and 6.0beta, for both Linux and Windows. In order to install it, select the "tools/plugins" menu, then the "downloaded" tabs and select the .nbm file you just downloaded. Install it, then restart SNAP. A new entry should appear in the "Optical" menu. Hover the mouse on the parameter fields of the new dialog for help, if needed. The graph processing framework is supported. Extra bonus: A batch version using Python and GDAL exists for use on clusters where SNAP cannot be installed, see the web page.
The method is quite computationally intensive. Please use a small test region of interest first, and only then expand according to your needs / available computing power.