How to use sed to replace a config file's variable?

I've been looking online for this answer and cannot seem to find it.

I have a config file that contains:

VAR1=VALUE1
VAR2=VALUE2
VAR3=VALUE3
VAR4=VALUE4
VAR5=VALUE5
VAR6=VALUE6

And I want to change VAR5's value from VALUE5 to VALUE10. Unfortunately, I do not know the value of VALUE5 so I cannot search for it. So basically I need to use sed (or whatever) to replace the value of VAR5 to another value.