If a pre-14.10 version of TTU is installed then please use the uninstall vb script to uninstall before installing 16.20. See Uninstall Teradata Tools and Utilities Pre-14.10 for details.
If a pre-15.10.10 version of TTU is installed, then those products have to be uninstalled before installing 16.20. This will be handled when installing the 16.20 Suite so it is not necessary to uninstall before installing 16.20. See Uninstalling Teradata Tools and Utilities 14.10 and Later.
If a 15.10.10 or later release is installed on the system then 16.20 can also be installed on the same system without uninstalling the 15.10.10 or later release. Please see Multi-Version Support for details.