Speed limits can only be updated in the Waze Map Editor (WME). Reports through the app are reported in WME as a User Request (UR) for an editor to review.
Currently only editors who are WME Beta users can review the SL reports.

You can always see when the last tile update was by looking at the status in the bottom left of the WME screen.

You can work with an editor who has access to WME Beta and your area to get the SLs updated if you choose.