Have a 625 with a 4" barrel. It has a gold bead sight, but I'm thinking of putting a tritium front dot on it. Is that somthing I can do myself or should I have someone do it?

If you can buy a correct front sight with the Tritium insert, it's a direct change-out.
Remove the sight blade pin and old sight, insert the new sight, and re-install the pin.

Things might get a little more involved if the new front blade isn't drilled for the pin.

If you can't buy an appropriate front sight with the insert, you'll have to have the insert installed by one of the night sight makers.
They all offer installation services, and in the case of a removable sight, all you would probably have to do is send in the actual blade for installation.

You cannot buy just the Tritium tubes to install yourself.
These are regulated by the Federal government and can be installed only by a company with the license to work with them.

Thanks for the input dfariswheel.