Cutting down shrubs
This type of pruning would be best done in late winter / early spring before the leaves come out.
I am unable to identify the plant from this photo to know if this is a spring flower shrub or not. If it is a spring flowering shrub, which blooms on previous year's growth, you will have fewer flowers next spring. However, full flowering should resume in subsequent year.
For additional information on pruning shrubs, please refer to the publication at the following link: https://njaes.rutgers.edu/fs1221/