How do i change the default temp? - Baxter OV300E Stainless Steel Electric Oven

The default for most Miele oven is 375 - however this is an earlier model oven that only allowed (as yo said) ten degree increments - I am afraid that there is no solution to that.
370 or 380 would be your options

to change between fahrenheit and celsius
The temperature is preset at Fahrenheit, but can be changed to
Celsius.
To Change: Press and hold CUSTOM BROIL for 5 seconds. A
tone will sound and “C” will appear on the display. Repeat to
change back to Fahrenheit. To remove, press UPPER OFF,
LOWER OFF or OFF/CANCEL. you can click on the link for guide http://www.ajmadison.com/ajmadison/itemdocs/8300609.pdf

To change: Press and hold PRECISE CLEAN for 5 seconds. A tone will sound, and "°C" or "°F" will appear on the display. Repeat to change back. To exit, press CANCEL/OFF.

you neglected to mention the model this is , but most of the ovens made by them , when switched to bake , 1st show the temp you adjust then after thrirty seconds or so show the actual temp of oven (this number will change) the set temp function can be adjusted to anything in the ovens heat range 100-450) using the arrow keys anytime in the bake cycle , just touching a arrow key will change display to set temp screen , from actual temp display

Seems all there is left to check is the switch

Your thermostat is STUCK. Replace. Your 'tripping mains' is your clue that your power is over loaded. Have an electrician check your boxes to make sure all is ok AFTER fixing stove.

Change the temp sensor.

After you hit the function you want...bake or broil...click the up arrow.  That should give you the default temp of 350 and the unit should light.  Now adjust to the temp you want.

what setting. the code to get into diagnostic area is 9428.

The 350 as you stated is a default set point that you see before you start the cycle. What you should see afterwards (start bake) is the current temperature - which would normally be lower than the set or desired temperature - it will eventually get there

Regards