Open Minecraft, press M (default Litematica menu key), and load your schematic.
> CONVERTING… 4128 blocks… 812 entities… 74 biomes… schematic file to litematica converter link
: A lightweight, open-source tool that supports conversion between .litematic PiTheGuy's SchemConvert on GitHub Requirements : Java 21 or later. Web-Based Schematic Converter Open Minecraft, press M (default Litematica menu key),
Converting building files between different formats—primarily from the older .schematic (MCEdit/WorldEdit) to the modern .litematic press M (default Litematica menu key)
To summarize:
A popular utility specifically designed to convert .litematic files back into .schem files for use with WorldEdit.