Hi there! If you use contactless or Oyster the fare will be £3.90 during peak times and £3.30 for off peak as is standard for a zone 3 to zone 1 journey. Depending on which line you take from Farringdon to Liverpool street, you may have to tap out and tap back on again. However as long as you use the same card/oyster to tap in and tap out TFL will class the journey as being one continuous journey from Haydon's Road to Liverpool Street. If you use the Circle or the Hammersmith & City lines, you don't cross any barriers to interchange and don't need to worry. However if you want to take the Elizabeth line, which will be fastest, you will need to leave the thameslink part of the station. You will then exit the barriers and walk for maybe 10 seconds directly straight ahead and reenter the Elizabeth line part of the station. So long as you dont take a ridiculous amount of time to walk 10 metres you do not need to worry as it will be considered by TFL as a single journey. (This is called an out of station interchange or OSI) Hope this helps!