Depends on how your site is implemented...
If it's a WordPress site, you can create a separate login for them, with their own password.
If it's a more traditional HTML site where you upload files via FTP, you can create a separate FTP login for them.
If it's some other system, there is probably an equivalent, you just need to create a login for them. You could also give them your login, but generally it's a good idea to have a separate one for them, just in case you have to disable their access for any reason.
For WordPress, you need to decide if they need admin rights, or editor -- depends on what they will be doing (editing content vs. working on design/infrastructure).
