FLT CREW; DURING ARR INTO LAS; IS ISSUED A RWY AND ARR CHANGE AND FAILS TO CORRECTLY PROGRAM THE FMC FOR THE NEW ROUTING.

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FLT HNL TO LAS. ON THE CLARR.CLARR1 ARR INTO LAS; APCHING MISEN; CLRED PLT'S DISCRETION TO 13000 FT. APCHING SKEBR; CLRED 'NOW TO CONTINUE VIA THE SKEBR2 ARR' AND TOLD TO EXPECT RWY 25R. I HAD PLANNED RWY 25L (ATIS). CAPT LOADED ARR AND RWY. JUST PRIOR TO SKEBR; BUT AT THE SAME TIME THE ACFT INITIATED ITS TURN TOWARDS ISSAR INTXN ON THE CLARR1 ARR; THE AMENDED RTE WAS EXECUTED. THEN IT SHOWED A CONTINUING L TURN BACK TO SKEBR. THEN ON TO KEPEC INTXN ON THE SKEBR ARR; I PUSHED HDG SELECT AND TURNED TO KEPEC AND THEN CLEANED UP THE RTE. I THEN TOLD THE CAPT I WOULD BRIEF THE APCH FOR RWY 25R. AFTER GETTING THE CHART OUT AND BRIEFING THE APCH; I SAW WE HAD NOT TURNED AT KEPEC TOWARDS IPUMY INTXN AND WERE NOW A COUPLE OF MI PAST KEPEC. AS WE STARTED TO TURN; THE CTLR HAD ALSO NOTICED AND ASKED IF WE HAD MISSED THE TURN TOWARDS IPUMY. RESPONDED YES; AND THE CTLR SAID NO PROB; WOULD VECTOR US FOR THE R DOWNWIND FOR RWY 25R. THE REST OF THE FLT CONTINUED WITHOUT FURTHER INCIDENT. SHOULD HAVE QUESTIONED ARR EARLIER AS 13000 FT IS NORMALLY GIVEN ON THE SKEBR ARR; FAILURE TO REARM LNAV AND FAILURE TO MAKE SURE A POSITIVE XFER OF CTL WAS GIVEN/ACCEPTED WHILE APCH WAS BRIEFED.