You need to set two properties Sequence
and TemplateAlias
. You cannot put the icon under the Delete item as it seems that the maximum icons you can put in this column is three. If you give Sequence="44"
and TemplateAlias="o2"
. This is what you will get:

However, if you set Sequence="55"
and TemplateAlias="o2"
. This is the result:

Full XML may look something like this:
<CommandUIDefinition
Location="Ribbon.ListForm.Display.Manage.Controls._children">
<Button
Id="Ribbon.ListForm.Display.Manage.Controls.ExportVersionHistory"
Alt="Export version history of the item to Microsoft Excel."
LabelText="Export"
Image16by16="/_layouts/15/images/NY.ExportVersionHistory/Excel_Small.png"
Image32by32="/_layouts/15/images/NY.ExportVersionHistory/Excel_32.gif"
Command="ExportVersionHistory"
Sequence="55"
TemplateAlias="o2"
ToolTipTitle="Export Version History"
ToolTipDescription="Export version history of the item to Microsoft Excel."/>
</CommandUIDefinition>