Select Read Only in the Collection menu to toggle the active collection between read only mode and read/write mode. The collection's tab will turn green when it's read only.
If you are the only person using a particular collection, making it read only does nothing except protect the collection from accidental changes. If you're accessing a collection via your local network while other people may also be using it, an additional factor comes into play: only one person can modify a collection at the same time.
The first person to open a collection will automatically open it in read/write mode by default. Everybody else will be forced to open the collection in read only mode, until the first person either closes the collection, or makes it read only. Then, another person can turn off read only mode for that collection and modify it.
If you turn on read only mode after you've modified a collection, AceText first saves your changes. When you turn off read only mode, AceText checks if the collection was changed on disk and automatically reloads it if it was. While working with a collection in read only mode, AceText will periodically check if it was modified by somebody else. The net result is that when collaborating across the local network, you don't have to worry about collections going out of sync or overwriting other people's changes.
If you use file synchronization software that updates your files in the background, it won't be able to update your AceText collection files that you have open in read/write mode in AceText. When synchronizing your files, make sure you don't have any collections open in AceText, or that the collections you have open are all in read-only mode.
In the Files Preferences you can enable various options to automatically make collections read-only in certain situations, or to open them as read-only by default. Use these options to give other people or other applications more opportunity to modify the collections that you have open in AceText.
There is no toolbar button for the Read Only command. If you've hidden the main menu, you can access the command by right-clicking on a collection's tab.