BACKUP your critical data
There is always a risk that the installation could cause errors. So BACKUP any thing which is important to you first. Because you are on the Mac you can zip up the directories and also backup on TimeMachine.
Update the Disc contents
1. Download the HardDisc4.zip file onto your RISC OS machine.
2. Open the file and run the Obey file InSituBootUpdate. The installer will backup the files in !Boot just incase.
3. Copy the contents for Apps, Diversions, Documents, Printing, Utilities into your copies to replace everything with new RISC OS 5.28 versions.
Upgrade RISC OS ROM
1. Download the RISC OS 5.28 stable ROM from the Download page. I downloaded the RiscPC version. The file is hidden away!
2. Copy the new files into RPCEmu Data directory and remove the old version (called ROM527 on my machine). Because I have asylum link to my Mac hard drive, I can do all this from inside RPCEmu.
I was then straight into RISC OS 5.28. I only wish updating my Mac to BigSur had been as quick and easy!