Yes. You may decrease your voluntary term life coverage amount during Annual Enrollment (to a minimum of $5,000). The decrease in coverage will be effective on the following Jan. 1.
Basic term life/basic AD&D coverage can only be decreased by reducing coverage amount to $50,000 to avoid imputed income, as outlines in IRS publication 15B here: https://www.irs.gov/publications
Voluntary AD&D coverage can be decreased by changing from a higher coverage level to a lower coverage level.
While changing your coverage amount can only be done during Annual Enrollment, you may cancel your voluntary AD&D and voluntary term life coverage at any time.