Yes, you may alter a template to fit your needs. You may not however, give this altered template to anyone outside of your household.
Every downloadable and custom template comes with a personal license, so you may modify and/or copy it, as long as it’s for yourself or your household. You may not give a template to anyone outside your household (unless you purchased a business license). This applies whether or not it’s a free or paid template, or whether the template is in its original format or has been modified.
Most templates have been password-protected to prevent accidental changes. You may need to unprotect the template first before you can make changes. To find out how to unprotect, check out this FAQ.