Ideally, global variables are static and can only be modified through UI.
You can use BotUserSession variables which will persist the value all over the bot for that specific user and session. These variables are dynamic.
Please refer the following document for more information on variables:
https://developer.kore.ai/docs/bots/bot-builder-tool/dialog-task/using-session-and-context-variables-in-tasks/
Let us know if you have any further issues/queries.
Regards,
Yoga Ramya.