The usual way is to go into Settings, find the "System Update" option (should be in System, or maybe About with some older versions) and select "check for updates".
A second method is to find a firmware file for the update and apply that manually. The depends on the manufacturer making these publicly available (or someone leaking them), and the method for applying them depends on the manufacturer (and I don't know the tools or method for Oppo). Note that in many cases this wipes the device, so best to check that first.
Of course both methods assume that an update exists. Since you don't say what version you are running or what/when your most recent update was we have no way of knowing whether you are already on the latest version available for that phone (updates are always device-specific, and have to be produced by the manufacturer. Budget devices generally get fewer updates, the really cheap ones generally never get any).