The reason why your Ryzen 5 3450U doesn't show up at AMD Download page it is because it is a OEM AMD Processor.
You will need to get the OEM AMD Graphics driver from your laptop's Support download page.
NOTE: You can try and install the Ryzen 5 3500U Mobile AMD Driver. It says it is compatible with Ryzen Processor with Radeon Graphic: https://www.amd.com/en/support/kb/release-notes/rn-rad-win-22-11-2
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from the release notes:
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If you decide to install the above driver make sure you first create a Windows Restore Point in case it messes up your laptop.
But it is best to use your laptop's OEM AMD Mobile Driver which is 100% compatible with your laptop.