The following will do a dry run, as it says. HEAD being current version, PREV is previous, then the path to your file, or committed item:
svn merge --dry-run -rHEAD:PREV https://example.com/svn/myproject/trunk
If the dry run looks good, run the command without the --dry-run
Verify the change in revision and re-commit. To browse for version numbers try:
svn log