There are two ways to do this:
1) You can assign the in-process PO to another user who has approval rights in ByD.
Go To: Application and User Management >> Task Distribution >> Business Task Management and then assign that item to a different approver.
2) You can setup a Task Substitution and assign a different user to act on behalf of someone who is on holiday or business travel. There is not effective date range for this change. You have to remember to switch the substitution off when the approver returns to the office. The Task Substitution is best when setup in advance of the approver's travel.
NOTE: You can also have the traveling approver do their approvals in the ByD mobile app when they travel. This isn't always convenient for people on holiday.
Jim