Modifying parameters in iApp via REST API

Your JSON body has a syntax error, make sure you enclose the whole statement in braces. Please note I've not been able to verify the API call is correct, but try amend the JSON body like below and let me know how you get on.

{"variables":[{"name":"maintenance__MAINTENANCE","encrypted":"no","value":"tech"}]}

If you're not using an IDE, this website is good for validating JSON https://jsonlint.com