At this time, the app does not offer the ability to extend a currently reserved job.
Our recommendation is to wait until you're able to complete the job before reserving the job. If you experience delays and cannot complete the job when you initially expected, we recommend canceling your reservation and searching for the job when you're ready again. Note that canceling a job will not negatively affect your account/score in any way!
Hopefully, another Agent doesn't get that job you were hoping to complete in the meantime!
Good luck out there!