You can cancel you current Short.io trial or subscription at any time by downgrading to the Free plan.
To cancel a trial/subscription
Sign in to your Short.io account.
In the left user's panel click on Subscription:
Click on Downgrade below the Free plan:
Hereafter, the features of your previous Enterprise, Team or Personal plan will be adjusted and limited.
Note: You can find more information on downgrading in the following article.