Z max pos doesn't change after uploading

I'm trying to change the z max position which is defined in marlin as z home pos. When j change this value to anything, it still always the z position after homing is 253.3 after homing but if I go to move z axis, i can increase to the value I defined in marlin but it just grinds against the endstops. Thanks for all the help recently from everyone

Again I don't use Marlin so I'm not totally sure but I've had a fiddle with the Marlin configuration tool here(N.B. I have no idea if that's recommended) and looking through the files it generated for me there's a line called

"deltaheight":

in the configuration.h file, whereas

Z_MAX_POS

only shows up in configuration_cartesian.h
So maybe you should see if you have that line in your files, and if not putting all your printer details into that config tool might get you what you need.

Asbo, thanks for the help. I've realised for some reason that problem is only when I use the lcd screen but now when I use repetier, it works fine and homes at the value I set but now when I jog z to 0, it moves down to 5mm above the bed and when I keep jogging down, the value on my laptop decreases but the extruder doesn't move

That might be caused by the printable height value, under printer settings in Repetier Host, being higher than what you've set in the firmware on the printer. This is a bit of a guess though.
You could try switching to Repetier firmware, the config tool is here. It's what I used before getting a Duet and it worked reasonably well.
If you bought your printer from someone or a company they might be able to help.
I was in a similar situation to you because I bought my printer as a kit from China, and whilst they were quite helpful the language barrier and timezone differences hindered things a bit. I also improved the printer as I built it so it didn't even match their spec any more