Avid Pro Audio Community - help with punch in PLEASE

Hi there ,
thanks taking time to read this , of course the answer to my question would be go and watch videos ,, but im having an issue understanding,
Ok im new to this , i have recorded wish you were here pink floyd ,, right i recorded main guitar part whole song no prob ,, on a seperate track i have done the lead ,, this is all good up untill my last bit of lead ,, now i dont want to keep retaking ,, i just want to highlight or however you do it the part i made a mistake in and just record that bit again ,, i go to my stem (is that what they are called ?) on my lead track try to highlight just the part i want to re record ,, it highlights the whole lot ,, ive tried putting a marker there ,, no difference ,, unfortunately i cant understand the instructions on how to do this ,, im hoping i can get help on here ,, think i will be on here alot ,, there are great tuition vids , but they are missing lots of vitals ,, Thanks again jim