As Pipeline said, click on F5.

But first, be sure you have the measure/bar selected where you want the change to occur.

Then after you click on F5, then click on the button near the top that says "RealTracks."

Then click on the Instrument Track that you want to change (note that these track names re the default/original track names and they will not reflect any previous RealTrack selections you have already made).

Then click on the down arrow at the far right side of the box under "Change RealTracks to..." and select the desired RealTrack. Note that they are listed in numerical order rather than being grouped by instrument. So all the variations of an instrument are NOT listed together. In the box above the list, you can enter a filter word to narrow it down. You can also enter the RealTrack number if you know it. For example, I frequently change horns to a string patch for a final held chord using 1841. Also, some sounds are louder than others, so you may need to enter a volume change for that instrument in the same measure. Changes can be + (louder) or - (softer).

After you make your selection, click on OK. Once you are satisfied, don't forget to save your edited song!

Jim
Psalm 33:3 Sing unto him a new song; play skillfully with a loud noise.

BIAB 2018 build 519; Dell Inspiron N4110, Windows 10 64 bit, Intel Core i3-2350m running at 2.29 GHz, Memory: 6 GB DDR3, SDRAM 1MHz