Is it safe to download a Malwarebytes update?

If you have Malwarebytes installed, which the message suggests, you can open that app and see if the app itself says an update is available:

If you have never installed Malwarebytes, it could be a scam so ignore. it.

how long till I expect a reply?

Patience. These are user-to-user forums, not Apple. We volunteer on a time-available basis so you simply have to wait until someone sees the question. Two things about getting responses in a timely manner:

1) Check the calendar. This is a weekend leading up to a major holiday season. Not as many people are hanging out here, rightly preferring to spend time with family and friends.

2) You posted in the forum section for reporting issues with the forum software, not for posting macOS questions. Fortunately I spotted this. I am asking the Hosts to move you to a more appropriate forum.

We are all volunteers here trying to help other users. There is no reply time expectation. But if you were to provide some context such as what app version to what app version and on what device and what operating system you may get a quicker reply.

dongail wrote:

Should I use this update? Is it safe to download?

Version Update Available

A new and improved version of Malwarebytes is available.

Malwarebytes is a trusted light weight utility that can be most helpful.

if in doubt go to the source: https://www.malwarebytes.com/mac/

Provided that you have already installed Malwarebytes, when a newer version is made available, there will be a notification panel that slides in informing you of this, and offering the option to download and install the update. This is how it works here. I normally click to allow it to update, because I have it installed, and have come to expect this behavior.