I have a trip FRA - SFO - 2 day layover - LAX. I migt want to change LAX to LAS but only after I land in SFO. Would it be entire ticket fare difference or just difference on SFO - LAS?

It depends on how it's ticketed. Could be a broken fare, could be a stopover. If the former, just the final segment would be repriced, if the latter, the whole ticket will be repriced (in which case it might be easier/cheaper just to buy a new ticket SFO-LAS).

Or you could see if you're able to SDC SFO-LAX to SFO-LAS-LAX and then just walk-away from the final segment :-)

Kacee is correct. You can determine which is the case by looking at your fare rules; if you have fare rules for FRA-LAX, you have a stopover, and the entire ticket would be repriced; if you have fare rules for FRA-SFO, and separately for SFO-LAX, you have a fare break at SFO, and only SFO-LAX would be repriced, although the entire ticket would still need to meet all applicable combinability rules.

However, I'd caution against planning just to walk away from the final segment unless this is the last flight coupon in the itinerary. If you have any further travel past LAX, it will be canceled if you fail to show up for your flight to LAX. In that case, you should compare the prices of changing to SFO-LAS vs. just adding an extra LAX-LAS segment. (Or, driving -- it's about a five-hour drive).

Thanks, unfortunatley my fare rules are only for FRA-LAX. SFO - LAX is my last segment. I tired to change flight now and United wants $2k fare difference LOL.

I tired to book separate ticket to LAS but was caught by United bot with overlapping travel times Finally I book alaska.

Better experience on AS anyway.