what should I reply

Hello,

What should I reply if somebody asks me this. "What you been up to ?"

Thanks in advance

The expression is very casual, short for "What have you been up to ?"  i.e. What have you been doing recently ?

You can give a reply according to how well you know the person, how much you want to discuss your activities, etc.  Your reply can be detailed, or perfunctory :  'I've been very busy at work lately',  'I've not had much time to do anything',  'I've not been doing a lot',  or just  'not a lot'.

what should I reply

Hello,

What should I reply if somebody asks me this, "What (there should be a "have" here, but I would understand if they didn't use one.) you been up to?"

Thanks in advance.

You tell them what you've done recently. Such as somethin that has happened at work or... Anything.
Example: Them: "What have you been up to?" You: "I just finished baking a pie." it really depends on what you want to tell them.

Something more serious; Example(2): Them: "What have you been up to?" You: "I've been working on this project at work that needs to be done in a week..."

Hope this helped!