Welcome!
How long have you had this quad? Has it ever shifted well for you? Did you select the down button while trying to get the up button?
Don’t know how old the battery is? I would load test the battery or, if you know it’s over three years old, buy a new OEM or equivalent.
If you’re positive battery is good, Sometimes you can free up a binding shift motor by adjusting the clutch as per service manual.
If these step don’t work there’s more.
Drain the fuel tank and try some ethanol free premium with a little Seafoam gas treatment
Let us know how if it worked and we’ll go from there