Violins I and II on a score.

First of all there is nothing wrong with Violin I and Violin II. I see this a lot. If you really want something like:

|-------- I
|Violin
|---------II

(Note: the | is there to keep the web page from reformatting the lines, and - represents a space)
then here's what you nee to do:

On the Violin II, delete the word violin and make it just say II for both the long and short names

On Violin I change the name to look like this:

|---------I
|Violin

You can press enter in the name and it will become part of the name. You will need to play with the spaces and proper number of enters to make it look right. The numbers of spaces and enters is dependent on to many variable that you may have changed in the Style settings. To make it easier, keep in mind that you can double click a name to put it in edit mode and immediately see what it will look like. Pressing Esc accepts your entry.

You can control the labels attached to any particular staff by opening the 'Staff Properties' dialogue (right-click on the staff, then left-click on STAFF PROPERTIES in the context menu which will appear). If you are writing both first and second violin parts on one staff (which is what I gather from your description, although it's not completely clear), delete the default label for that staff in both 'long instrument name' and 'short instrument name' boxes, then enter:

Violin I
Violin II

in the 'Long instrument name' box, and

V I
V II

in the 'Short instrument name' box. In both cases, use a hard return between the two lines of text.

The 'long instrument name' appears at the beginning of the first system in the score; the 'short instrument name' appears at the beginning of each subsequent system.

If you are trying to do something else, please attach the score in question (or a PNG snippet of it showing the first system of the score) so we can see what your problem is.