How to upgrade Windows 8 pro OEM to 8.1 pro for a home PC builder

Question

Built a PC. bought Windows 8 Pro OEM and installed.  don't need retail as I have never needed support before.  Tried to upgrade to 8.1 but my licence is OEM (I have a COA), but it will not update through store.  Advised that 'volume licences'
must consult system administrator.  As I have agreed to the home PC builder agreement, then I am the administrator.  Check through Microsoft and for Volume licences you can down load 8.1 upgrade from the Microsoft Volume Licensing Service Center
(VLSC), but I have a single OEM licence so cannot access that route.
Can I ask if there is a way to upgrade to 8.1 or do I have to buy windows 8.1 OEM and install that. 

Answer 1

COA Sticker says 'Windows 8 pro for oem software' then the product key
<DiagReport>
<LicensingData>
  <ToolVersion>6.2.9200.16384</ToolVersion>
  <LicensingStatus>SL_LICENSING_STATUS_LICENSED</LicensingStatus>
  <LicensingStatusReason>0x00000000</LicensingStatusReason>
  <LocalGenuineState>SL_GEN_STATE_IS_GENUINE</LocalGenuineState>
  <LocalGenuineResultP>1</LocalGenuineResultP>
  <LastOnlineGenuineResult>0x00000000</LastOnlineGenuineResult>
  <GraceTimeMinutes>0</GraceTimeMinutes>
  <TotalGraceDays>0</TotalGraceDays>
  <ValidityExpiration></ValidityExpiration>
  <ActivePartialProductKey>PWCK2</ActivePartialProductKey>
  <ActiveProductKeyPid2>00178-50102-15414-AA046</ActiveProductKeyPid2>
  <OSVersion>6.2.9200.2.00010100.0.0.048</OSVersion>
  <ProductName>Windows 8 Pro</ProductName>
  <ProcessorArchitecture>x64</ProcessorArchitecture>
  <EditionId></EditionId>
  <BuildLab>9200.win8_gdr.140502-1507</BuildLab>
  <TimeZone>GMT Standard Time(GMT+01:00)</TimeZone>
  <ActiveSkuId>949d6b86-bfa7-4ff1-b4df-17e67bb6320d</ActiveSkuId>
  <ActiveSkuDescription>Windows(R) Operating System, VOLUME_MAK channel</ActiveSkuDescription>
  <ProductUniquenessGroups>55c92734-d682-4d71-983e-d6ec3f16059f</ProductUniquenessGroups>
  <ActiveProductKeyPKeyId>86202a92-0652-8022-5991-343ba2f20bc2</ActiveProductKeyPKeyId>
  <ActiveProductKeyPidEx>05426-01785-010-215414-03-2057-9200.0000-2452014</ActiveProductKeyPidEx>
  <ActiveProductKeyChannel>Volume:MAK</ActiveProductKeyChannel>
  <ActiveVolumeCustomerPid></ActiveVolumeCustomerPid>
  <OfflineInstallationId>566732249641728780885422514181499647154083204397030748926614966</OfflineInstallationId>
  <DomainJoined>false</DomainJoined>
  <ComputerSid>S-1-5-21-2653051395-138031257-847549496</ComputerSid>
  <ProductLCID>2057</ProductLCID>
  <UserLCID>2057</UserLCID>
  <SystemLCID>2057</SystemLCID>
  <CodeSigning>SIGNED_INFO_PRS_SIGNED</CodeSigning>
  <ServiceAvailable>true</ServiceAvailable>
  <OemMarkerVersion></OemMarkerVersion>
  <OemId></OemId>
  <OemTableId></OemTableId>
  <Manufacturer>Gigabyte Technology Co., Ltd.</Manufacturer>
  <Model>To be filled by O.E.M.</Model>
  <InstallDate>20140902153518.000000+060</InstallDate>
</LicensingData>
<HealthCheck>
  <Result>PASS</Result>
  <TamperedItems></TamperedItems>
</HealthCheck>
<GenuineAuthz>
  <ServerProps>GenuineId=55c92734-d682-4d71-983e-d6ec3f16059f;OemId=;OptionalInfoId=;Pid=6Ydd8896JiOFO1zSH32MVg40Lh+PZGZaDEGDuuX+bwg=;SkuId=949d6b86-bfa7-4ff1-b4df-17e67bb6320d;TimeStampServer=2014-09-02T19:07:22Z;</ServerProps>
</GenuineAuthz>
</DiagReport>