Hi,
Go to your report's paper format and set header_line as False.

if you have not created a custom paper format create a custom one and set it in your report
like this, after that, you can set attributes as you need. 

<record id="paperformat_custom"  model="report.paperformat">
 <field name="name">A4 Custom</field>
 <field name="default" eval="True"/>
 <field name="format">A4</field>
 <field name="page_height">0</field>
 <field name="page_width">0</field>
<field name="orientation">Portrait</field>
 <field name="margin_top">5</field>
<field name="margin_bottom">13</field>
 <field name="margin_left">7</field>
 <field name="margin_right">7</field>
 <field name="header_line" eval="False"/>
<field name="header_spacing">15</field>
 <field name="dpi">90</field>
</record>

Hope it helps,

Hi 

You have to edit  the top margin in the paperformat of the report . 

Go to settings > Technical > Paperformats 

Choose the Paperformat and edit the Top margin .

Regards