"Change Icon" dialog is available in Customization dialog's Toolbars tab, Assigned Commands list (right-click an item, or select an item and press the "Modify" button below).

The "Change Icon" help page [1] needs improvements:

1. It must state that it assigns only the *current* size of icon to the element. The background: there are (currently) three icon sizes for toolbars, selectable from Options->View: small (16x16), large (24x24), and extra large (32x32). All three may be assigned to a command on a toolbar; but the UI allows to do it only in a unobvious way: one needs to set the toolbar icon size to small in Options, then assign small icons; then set the toolbar icons to large, then set the large icons; and finally, set the toolbar icons to extra large, then set the extra large icons. And this means, that when you call the "Change Icon" dialog, it works with the currently selected icon size.

2. There is a phrase there: "You can only import icons that are in the PNG file format and that are 16 × 16 or 24 × 24 pixels in size". This phrase is wrong for two reasons: first, it tells about only two sizes, while, as said, currently there are three icon sizes (extra large icon size was added in version 5.3: https://wiki.documentfoundation.org/ReleaseNotes/5.3#Icons); and second, user may choose an image with *any* size, and *any* file format - it will be scaled to the current size, and converted, automatically.

[1] https://help.libreoffice.org/24.8/en-US/text/shared/01/06140402.html