Can I change the available time during a sprint?

Yes, you can. This works best if you have specified the available time individually for each day in the sprint, since you can then change the available time for each remaining day in the sprint:

If you have specified the available time as a total for all days, and you change the total time, the ideal line (the gray dotted line) in the burndown chart will distribute that time across all days in the sprint. This is probably not what you want. In that case, you can simply switch to specifying the available time individually for each day, and then change the available time for each remaining day in the sprint. To do that, open the popup dialog for the team in the sprint, and click the link "Specify for each day instead":