I have a client who had a design they had done by another artist and wants me to finish up the logo using what they designed, im just worried legally if i can use what they did or what i need to get from the artist before making changes. Any suggestions?
This depends entirely on whether the previous designer was employed to do "work for hire" or if there's other contract language granting client full IP rights to work, including derivative works which is what your "completing" their previous work would legally be.
If there's no explicit wording granting IP rights to client, they don't own any of it at all, and you're better off not using that starting point.
If the client or your working relationship with the previous designer is not in tatters, contact them and find out what the IP status is for that work - if they are on reasonable terms with client, they may be willing to issue a rights-granting for "completion". If however the client has failed to pay for work tendered, the previous designer would be fully within their legal rights to refuse to grant IP rights - and if that is the case, you should be very careful in contract dealings with said client.
I'm not a lawyer at all, and this shouldn't be taken as competent legal advice per se, but I am a multi-decade graphic designer & illustrator, so I am somewhat familiar with the topic at hand.
Nothing in this answer shall be considered legal advice and no attorney-client relationship has been established.