The MKVToolNix GUI way:
- From the menu choose header editor.

Open the mkv file.
Under segment information there is a title item, change title as you wish.

- There is not a save button, use the main menu to save changes.

macOS Version Support
Current versions of MKVToolNix require macOS Mojave (10.14) or newer, but earlier versions of MKVToolNix might work for previous versions of macOS/OS X.
As of v42.0.0, MKVToolNix uses std::optional,a which requires macOS 10.14.b
macOS High Sierra (10.13) and Sierra (10.12) should run up to v41.0.0 "Smarra" (2019-Dec-06).
As of v30.0.0, MKVToolNix uses Qt 5.12,c which requires macOS 10.12.d
OS X El Capitan (10.11) should run up to v29.0.0 "Like It Or Not" (2018-Dec-01).
As of v26.0.0, the "README.macOS.txt" file states that it works only with macOS 10.11 "El Capitan" or newere (although MACOSX_DEPLOYMENT_TARGET and LSMinimumSystemVersion are still at 10.9, so it might run in Yosemite 10.10 or Mavericks 10.9).
OS X Yosemite (10.10) and Mavericks (10.9) should run up to v25.0.0 "Prog Noir" (2018-July-12).
As of v9.7.1-build4, MACOSX_DEPLOYMENT_TARGET and LSMinimumSystemVersion are set to 10.9.f
OS X Mountain Lion (10.8) might run up to v9.7.1-build2 "Pandemonium" (2016-Dec-27).
As of v8.2.0, the "README.macOS.txt" file states that it works only with Mac OS X 10.9 and higher, although LSMinimumSystemVersion is still at 10.8.0, so it might run in Mountain Lion 10.8.
Furthermore, MACOSX_DEPLOYMENT_TARGET is not set, so it should target whatever OS version it was built in.g As v9.7.0-build2 and v9.7.1-build2 were built in Sierra 10.12, they won't run in Yosemite 10.10 nor El Capitan 10.11 (although they will run in Mavericks 10.9 and Mountain Lion 10.8, which appear to ignore that restriction). See the relevant discussion here.