If you want to revert everything to a previous version (ie revision 72):
svn merge -rHEAD:72 .
Note the “.” at the end!
If you want to revert all local changes (ie say go back to the head revision):
svn revert -R .
Again, note the “.” at the end.
To view the diff between two versions (ie what changed between revisions 72 and 73), use:
svn diff -r 72:73 file.java