Switch to first tab in Notepad++

Press Alt+W to open the Window menu

Press 1 to open the first window, which is the same as the first tab.

Use Ctrl+tab it will show the list of tabs available, keep pressing tab without releasing ctrl key to cycle between tabs list, else click on the tab name to switch to it.

You could install AutoHotKey and use the following script - this is what I've done myself. It allows you to use ALT-1, ALT-2, ALT-3, etc. to choose those numbered tabs quickly, by automating what Wayne Johnston suggested.

#IfWinActive ahk_class Notepad++
{
    !1::SendInput !w1
    !2::SendInput !w2
    !3::SendInput !w3
    !4::SendInput !w4
    !5::SendInput !w5
    !6::SendInput !w6
    !7::SendInput !w7
    !8::SendInput !w8
    !9::SendInput !w9
}

Believe me, once you start using AutoHotkey, a whole new world of automating and customising your apps' keyboard usage opens up. It's even kind of addictive once you get into it. :)