Where to specify a previously installed racket version or...

[infodeveloperdon]

Where to specify a previously installed racket version or...

is the only way to change back from version 7.8 to 7.7 to re-install version 7.7?

[George Neuner]

You can install multiple versions in parallel as long as they are kept
aprat in separate directory hierarchies.  This is the default on
Windows, but on Linux be sure to choose an install option that keeps
everything together and doesn't create extraneous links in the top level
system directories.

George