I don't know if there are Ubuntu packages available, but if not, you could build from source and install to
/usr/local. This means you'll end up with 8.5 (from the distro) and 8.6 installed. To favour 8.6 by default, make sure
/usr/local is in your ld.so.cache; I think ubuntu does this by default. To check:
grep "/usr/local/lib" /etc/ld.so.conf.d/*
If you get nothing, create an
/etc/ld.so.conf.d/local.conf with one line,
/usr/local/lib. In either case, run
ldconfig (presuming you've already built and installed tcl/tk).
LD_LIBRARY_PATH=/usr/local/lib to your default environment. This will make the linker prefer those libraries to the distro ones, which is fine unless you have previously done wonky things in /usr/local. You'll also need
/usr/bin in $PATH, if it is not that way already.
You should now default to using 8.6. Leaving 8.5 installed is just a lazy way of ensuring the package manager has it for dependencies.