In registry editor change the key:

HKEY_CLASSES_ROOT\textfile\shell\open\command

and add or modify a value: Type REG_EXPAND_SZ

"c:\tools\notepad++\unicode\notepad++.exe" "%1"

the reg file is:

Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\Applications\notepad++.exe]
[HKEY_CLASSES_ROOT\Applications\notepad++.exe\shell]
[HKEY_CLASSES_ROOT\Applications\notepad++.exe\shell\edit]
[HKEY_CLASSES_ROOT\Applications\notepad++.exe\shell\edit\command]
@="\"C:\\Program Files (x86)\\Notepad++\\notepad++.exe\" \"%1\""
[HKEY_CLASSES_ROOT\Applications\notepad++.exe\shell\open]
[HKEY_CLASSES_ROOT\Applications\notepad++.exe\shell\open\command]
@="\"C:\\Program Files (x86)\\Notepad++\\notepad++.exe\" \"%1\""