How to modify a Postgres tablespace location
The tablespace location is defined by a symbolic link in the pg_tblspc directory (this is why, by the way, you can only use custom tablespaces on systems that have symlinks, as stated in the documentation):

~/9.5/main/pg_tblspc$ ls -l
total 0
lrwxrwxrwx 1 postgres postgres 13 Mar  5 10:33 132568 -&gt; /tablespace2/


As user postgres, you can change the location this symlink points to:

postgres@local:~/9.5/main/pg_tblspc$ ln -snf /tablespace/ 132568


This will fix your tablespace location.  As a precaution, I'd do this only when the database is not running.

However, you have to be sure why the location changed in the first place and that it won't change back or to anything else.