How to use/change adjustable heater

Currently I have a preset heater in a 33 gallon tank with a betta in it. It maintains a temp of 75-76 degrees but I would like to switch it out for an adjustable heater so I can set the temp to 79-80 degrees.
This is the heater I got.




I don’t know how to go about switching the heaters, any advice?
Should I set this new heater to 75 while the preset one is running or would that over heat the tank? Should I take out the old heater and set this one to 79?

 

I don't think you have to worry about overheating the tank with both heaters in there, once the desired temp is reached the heaters automatically turn down.

I just recently switched from preset to an adjustable 100w in my 30 gallon. I put the adjustable heater in with the preset one. I sat the adjustable one just a degree or two higher than what the tank was reading. When the new heater turned green I took out out the preset heater. I then upped the new thermometer about a degree an hour until I got the tank where I wanted it.

 

I don't think you have to worry about overheating the tank with both heaters in there, once the desired temp is reached the heaters automatically turn down.

I just recently switched from preset to an adjustable 100w in my 30 gallon. I put the adjustable heater in with the preset one. I sat the adjustable one just a degree or two higher than what the tank was reading. When the new heater turned green I took out out the preset heater. I then upped the new thermometer about a degree an hour until I got the tank where I wanted it.

I think the problem with mine is that my tank is at 75, and the adjustable heater goes from 75-79. I don’t think I can put it in the middle

 

Found this video on youtube, looks like you will have to guess a bit but that you can slowly dial it up and down. I would keep an eye on the thermometer to make sure it's increasing at a nice slow gradual pace.

 

Found this video on youtube, looks like you will have to guess a bit but that you can slowly dial it up and down. I would keep an eye on the thermometer to make sure it's increasing at a nice slow gradual pace.

Thank you so much!