Change style of measure numbers

Two choices to do it...

1. Use menu item: Format > Style > Text styles > Measure number -- where such edits can be made globally (i.e., for the whole score).
2. You can click a measure number and make the change in the Properties panel (F8). This will apply any change to the selected measure number only. To apply the change globally to the whole score, you can click on the ellipsis (…) above the property you changed and select "Save as default style for this score". Doing so will update the (global) value found in 1 above.