B757-200
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Total 17000
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Procedure
Description: While on the ZZZZZ arrival we were told that we would be doing the ILS XX when we were expecting the ILS XY. I inserted that approach in the CDU (Control Display Unit) and then began briefing the approach. While in the middle on the brief the First Officer stated that the aircraft did not make the turn at the ZZZZZ1 intersection. I looked at the AFDS (Automated Flight Director System) panel to see what mode we were in and we were indeed in LNAV and it should have made the turn. I told the FO (First Officer) to turn the plane right when ATC called and told us to do the same thing. I then noticed on the Progress Page 2 that the Xtrack error line was blank. The legs page also showed the active waypoint as ZZZZZ2 which was behind us. I did not change the arrival in the CDU; only the ILS. I do not know why the legs changed the way that it did. Cause: The runway change caused us to make changes in the CDU and which took away from my ability to monitor the aircraft's navigation.Suggestions: Suggest not making changes in the CDU whenever the aircraft is approaching a turn or other required change in the flight's altitude or speed. This will allow the crew to verify the changes are correct before any of these changes are needed. This will also allow the crew to better monitor the aircraft during these lateral; vertical and speed changes.
B757 Captain reported clearance heading deviation after changes were made on the CDU on approach.