As or upgrading to 0.5.X, you should be able to go to the settings menu and click the update button if you are on 0.4.18. https://github.com/getumbrel/umbrel/issues/1422

If not, then I'm not then in all honesty not sure what the procedure is. It might let you update to 0.4.18 and then after 0.5.X.

If not, you may need to manually apply the fix or reinstall 0.5.X.