UnselectRows(
    visibleIndices?: number[] | number
): void

<dx:ASPxGridView ID="MyGridView" ClientInstanceName="grid" runat="server"
    OnContextMenuInitialize="GridView_ContextMenuInitialize">
    <ClientSideEvents ContextMenuItemClick="OnContextMenuItemClick" />
    <%--...--%>
    <SettingsBehavior AllowSelectByRowClick="True" />
    <SettingsContextMenu Enabled="True" />
</dx:ASPxGridView>

function OnContextMenuItemClick(s, e) {
    switch(e.item.name) {
        case 'SelectAll':
            grid.SelectRows();
            break;
        case 'DeselectAll':
            grid.UnselectRows();
            break;
    }
}

protected void GridView_ContextMenuInitialize(object sender, ASPxGridViewContextMenuInitializeEventArgs e) {
    e.ContextMenu.Items.Add("Select All", "SelectAll");
    e.ContextMenu.Items.Add("Deselect All", "DeselectAll");
}

Protected Sub GridView_ContextMenuInitialize(ByVal sender As Object, ByVal e As ASPxGridViewContextMenuInitializeEventArgs)
    e.ContextMenu.Items.Add("Select All", "SelectAll")
    e.ContextMenu.Items.Add("Deselect All", "DeselectAll")
End Sub

<dx:ASPxGridView ID="ASPxGridView1" ClientInstanceName="grid" runat="server">
    <%--...--%>
</dx:ASPxGridView>


<dx:ASPxButton ID="ASPxButton1" runat="server" AutoPostBack="False" Text="Unselect row with index #2">
    <ClientSideEvents Click="UnselectSecondRow"/>
</dx:ASPxButton>

function UnselectSecondRow(s, e){
    grid.UnselectRows(2);
}

<dx:ASPxGridView ID="ASPxGridView1" ClientInstanceName="grid" runat="server">
    <%--...--%>
</dx:ASPxGridView>


<dx:ASPxButton ID="ASPxButton1" runat="server" AutoPostBack="False" Text="Unselect row with indices ##1-3">
    <ClientSideEvents Click="UnselectMultipleRows" />
</dx:ASPxButton>

function UnselectMultipleRows(s, e){
    var indices = [1, 2, 3];
    grid.UnselectRows(indices);
}