To send a reminder to a reviewer that is listed as ‘Pending’ i.e. they have not yet accepted your invitation to review, you can simply re-invite them following the guide outlined here.
If you are looking to extend the deadline for the reviewer, this can easily be done by clicking on the intended reviewer on their name on the right-hand side of your ‘Papers’ tab, as before:
From here, you can view and edit the deadline by which they should upload their review of the paper(s):
Once you have selected your new deadline, the reviewer will have until the updated date to submit their review of the paper.