The two sentences mean different things
1.
To solve a complex problem, there is always a simple way which
everyone can understand.
This says that everyone can understand the way. It is probably what you are wanting to say and it is grammatically acceptable in British English.
However, in American English, they make a distinction between "that" and "which". You sentence should then be:
To solve a complex problem, there is always a simple way that everyone can understand.
I would prefer this last version in both American and British English.
2.
There is always a simple way to solve a complex problem, which
everyone can understand.
This has a totally different meaning, especially in American English. It means:
Everyone can understand that there is always a simple way to solve a complex problem.
The most difficult aspect of your sentence to interpret is that it is not true! There aren't always simple solutions to problems!