Go to Edit mode, Then UV Editing
In Data, Choose UV map and click - sign to remove old UV map
Select all vertix of object (hit A),
hit U or UV icon on top, Unwrap
Click +New or open the image texture, set as follow if choose new
Go to Shading,
add new material, select the BSDF and hit Ctrl T (create image texture, mapping and coordinate nodes)
Activate Node Wrangler Add on
Add image by clicking the node below, select the image
Change the 3D persepective interface to Material Preview mode to see the texture just added on the object
Select with edge nodes to select which parts want to separate as independent UV map
Use Mark Seam (show with red line to recognize)
After manually improve the default mapping,
Select all in 3d perspective and click Unwrap again.