Normally, Membership Pro only allow upgrading subscription from plan A to plan B if the current subscription of plan A is still active. With this override, the system will allow upgrading even if the subscription for plan A (free) was expired before
You just need to get this file, unzip it, upload the received file to components/com_osmembership/helper/override folder on your site to have it works
Sorry this is now URGENT> I tried uploading the file directly to helper folder and that didn't work. So I went to delete the file and I accidentally deleted a different file by mistake and now my site is throwing this error. I also can't navigate away to anywhere else in the admin including eXtplorer or acybackup or even LOGOUT! and now my site is completely screwed up with even the home page not showing. I made a backup just before trying this.