Under Joomla 3
Go to Extensions -> Manage -> Manage. Type 'Truly Responsive' in the search box.
The free version
Select the 'Truly Responsive Slides' package and click on Uninstall.
The pro version
Select the component ONLY and click on Uninstall. This will remove all extensions at once. If it does not, remove them one by one.
Joomla 3.10 You may see an additional Package called 'Truly Responsive Slides Pro'. Remove the package instead of the component (all extensions tied to the package can't be removed individually).
Under Joomla 4
Go to System. In the Manage section, go to Extensions.
Type 'Truly Responsive' in the search box.
The free version
Select the 'Truly Responsive Slides' package and click on Uninstall.
The pro version
Select the 'Truly Responsive Slides Pro' package then click on Uninstall. This will remove all extensions installed through that package.