<!-- Core -->
<?xml version="1.0" encoding="utf-8" ?>
<Patch>
    <Operation Class="PatchOperationReplace"> // Replace since you want to replace
        <xpath>/Defs/GeneDef[defName="FurryTail"]/iconPath</xpath> // look in the xml file and write the path to the node you want to modify starting from the Defs node
        <value>
            <li>your path</li>
        </value>
    </Operation>
</Patch>


<!-- Modded -->
<?xml version="1.0" encoding="utf-8" ?>
<Patch>
    <Operation Class="PatchOperationFindMod"> // "if mod is loaded"
        <mods>
            <li>Monster Girl Encyclopedia Inspired</li> // mod name, not id, as according to its about.xml
        </mods>
        <match Class="PatchOperationSequence"> // let you declare a list of patch operations like above in order of execution
            <operations>
                <li Class="PatchOperationReplace"> // like above, only the Operation tag is changed
                    <xpath>/Defs/GeneDef[defName="MGEI_AlrauneFlower"]/iconPath</xpath>
                    <value>
                        <li>your path</li>
                    </value>
                </li>
            </operations>
        </match>
    </Operation>
</Patch>