Can I increase/decrease the trial period for a particular customer?

Yes, you can.

While creating a new subscription for a customer, in the Trial Days field you can enter the number of days you want to provide the trial period of the plan for the customer.

For the subscriptions created already,

• Navigate to Subscriptions tab and select the subscription you want to change the trial period of and click on Edit.
• In the Trial Days field, enter the number of days you want to increase or decrease the trial period.