Update value of custom user field from external application

I have a custom profile field setup to take integer values, but this value will need to be automatically changed by my external Laravel application, whether that is via. a POST request to my WoltLab installation or a cronjob running a check on the external database ever so often, and doing a cross check between the external app's database value and the user field value, updating if necessary.

Does anybody have any suggestions as to how I should/can go about doing this? Thanks