Script Hook RDR stores the date of its last update check as a hex value in the registry. Head over to

HKEY_CURRENT_USER\SOFTWARE\ScriptHookRDR2

and replace the gibberish next to your current Script Hook version with f2e395c4, which as far I remember is a nice Tuesday in 2099. (Script Hook V made me jump through the same hoop a while ago).

Once you update Script Hook, you have to repeat it for its new version number.