Can I change the unit used for estimates?

Yes, you can configure the units to use for rough estimates and detailed estimates. To change the units for an existing project, open the project dialog (in the Projects page) and click "Settings" in the upper right corner of the dialog. Then change the "Unit for rough estimates" or "Unit for detailed estimates" setting:

You can use points, days, or hours. If you've already entered estimates somewhere in the project, you'll have the option of adjusting those estimates as part of changing the unit, such as multiplying by 6 or 8 to go from days to hours.

If you are using detailed time tracking, you can also change the unit used for the time tracking, in the project dialog.

If you're administrator, you can also change the default units used in new projects. This is done in the "Your company account" page in the settings:

Note that the default units only affect new projects created after your change, they do not affect the units used in already existing projects.