You just have to change the api version in the plugin.yml. But beware, do this only if you know how to fix the coming errors by the new api
you must use a devtools plugin: https://poggit.pmmp.io/ci/pmmp/PocketMine-DevTools/PocketMine-DevTools
you mean after changing the plugin to 3.0.0-ALPHA9 so to put his folder inside devtools folder that inside plugins folder?
No, 1.Download your DevTools plugin and paste it into your plugins folder of your server 2.unpack your zip plugin 3. Move the newly unzipped folder into the server plugins folder 4. restart your server