You can actually use an SQL editor, I use DB browser SQlite. (https://sqlitebrowser.org/dl/) ?
Go to Server\resources, then locate the playerdata file.
Browse data tab > Table: Players > 13th and 14th options are XP and Level, you can just edit the values to 0 in the edit database cell on the right.
then control+S write changes, and close.
you probbably can run SQlite while the server is active as long as you arent doing anything while its open, close SQlite before you run around in the game world or it will crash the server, probbably best to do SQlite editing while the server isn't running tbh.