To switch the currently selected toolchain version (the one with the *), run the nrfutil toolchain-manager install and specify the installed toolchain version you want to use with the --ncs-version flag, just as if you were installing it.
For example, the following command switches to using the v2.6.1 of the toolchain:
$ nrfutil toolchain-manager install --ncs-version v2.6.1
[00:00:00] ###### 100% [Install toolchain] Toolchain with version v2.6.1 already installed