In my attributes table I want to insert a new value to a few selected items, and keep the old data inside the field. The table contains multiple columns (street name, length... and public transportation). I have selected a few roads and I need to update them by adding a new public transportation (,bus 332). How can i achieve this?
If you mean update a field in multiple selected records (rows), you can do that in Field Calculator. If you have selected records, Field Calculator will work on just those records.
To keep the existing values and add '.bus 332', in the MYFIELD = box, you can use something like MYFIELD & ".bus 332", where MYFIELD is your field name. Note, this works on one field (column) at a time.
If you have multiple fields, not (just) multiple records you want to do this on, you'll have to do it individually or use a script to do so. I'm not clear on which scenario you mean.