https://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-dug-propertypage.html
Adding and Editing Properties
Figure 4.36. Adding properties
To add a new property, first click on New....
Select the required property name from the menu, and then
fill in the required information in the specific property
dialog. These specific property dialogs are described in more
detail in the section called “Property Editors”.
To add a property that doesn‘t have its own dialog, choose
Advanced from the New...
menu. Then either select an existing property in the combo box
or enter a custom property name.
If you want to apply a property to many items at once, select the
files/folders in explorer, then select
Context menu → properties.
If you want to apply the property to every file
and folder in the hierarchy below the current folder, check the
Recursive checkbox.
If you wish to edit an existing property, select that property from
the list of existing properties, then click on Edit....
If you wish to remove an existing property, select that property from
the list of existing properties, then click on Remove.
The svn:externals property can be used to
pull in other projects from the same repository or a completely
different repository. For more information, read
the section called “External Items”.
Edit properties at HEAD revision
Because properties are versioned, you cannot edit the properties
of previous revisions. If you look at properties from the log
dialog, or from a non-HEAD revision in the repository browser,
you will see a list of properties and values, but no edit controls.