Is this an application running in the Mendix cloud? I think the heap space depends on the cloud node you ordered. If you need more, you probably need a biger cloud node.
You should ask Mendix Support for mor information.
If you have deployed in the Mendix Cloud, you will not be able to adjust this: the amount of memory your application has access to is governed by your subscription. If you want to increase that, you should contact your customer success manager, or submit a ticket at the support portal (NB: this will incur a monthly fee).
If you deploy on premise, you should refer to the documentation here.