War -> Garrison Units -> Deploy Garrisons.
Click on the targets for which you want to dismantle the garrisons.
If the capital, for example, click on 'Capital', then under 'Defense based on (blueprint)', pull the box down, select 'Undefended', and push the button 'Deploy Garrison Units'.
You can dismantle several garrisons in each section at a time. If you're dismantling the garrisons for all your forts, for example, go to the 'Forts' section, put in 'Undefended' at the top next to 'For selected Fortifications', and click 'Apply to current selection'. After it propagates the changes to all the selected forts below, you still need to select the 'Deploy Garrison Units' button to make it effective.