Hi Kay
It's not possible to select this on your profile. You will change from 'junior member' to 'member' when you have made 50 posts on the Community, then rise to 'senior member ' when you have made 250 posts. We only have two senior members at the moment - Nixon and JustAndy103 - and Mick W and LovelyLesley aren't far off.
But I wouldn't worry too much about it - it doesn't affect what you can do on the Community.
Kind regards,
Olly, your editor