If it's a week or more before the race
Go to https://theentrypoint.co.uk/, choose 'Sports Events' from the 'Categories' list at the top, then find the Rushmoor Wellesley 10k race, and click on it to open the event page.
On the right of the event web page, under 'Information' click on 'Edit Details and Transfer Entry'
Login by entering your email address and Unique Reference Number. If you don't have a URN, click on 'Forgot Reference Number' and enter your email address to get it sent to you, then you can login
Edit your details.
If it's close to race day but before Saturday
You should use the race web site Contact Us form supplying any updates to your Name, Title, Date of Birth, Sex, Club (if any).
If it's the Saturday before the race or later
It may be possible to get your details updated after the race, before the results are published. You need to supply any updates to Registration but the team will be very busy so no guarantees!