I’ve got a secondhand UM2+, but it shows poor printing quality. For example, the printed objects have voids on the surface, and it prints ovals instead of circles. I think I might need to replace some parts to fix these issues, but I have no idea what should be replaced because I just started using 3D printers. Any tips would be appreciated!
I wouldn’t rush into replacing parts. Adjusting your print settings doesn’t cost anything and will likely lead to a big improvement. You would only look at replacing things when you have pinned the problem down to the precise component.
The bubbles and blobs are likely due to the filament absorbing moisture, especially if it’s PLA. Are you using the second-hand filament that came with the printer, or is this a new roll? The old filament can be dried by gently heating in an oven. How to Dry Filament: PLA, ABS & Nylon | All3DP
Circles printing as ovals could be an error in the machine configuration - specifically in the number of steps per mm for the axes. You can identify that problem by printing off a suitable test piece. Benchy is a good demonstration piece, but there are others that identify particular problems. The Top 15 Free 3D Printer Test Print Models | All3DP
At a deeper level of detail, your slicer will have a myriad of settings that can affect the print quality, and there is a mass of resources available for adjusting the settings to deal with specific problems. Exactly how you do this depends on the slicer you are using.
I agree whole heartedly with Jeff’s advice here, the only thing I’d add is that Teaching Tech, and Aussie 3D printing youtuber, has put together a good site and companion video for things to check on when you get a new printer, or things aren’t working the way you expect: