Select the object
Apply scale with Ctrl+A if needed
Go to Object Mode
Add a Decimate modifier
Choose Collapse, Un-Subdivide, or Planar
Adjust the Ratio or Angle Limit
Apply the modifier when satisfied
Use Limited Dissolve in Edit Mode for flat areas
Dissolve edges manually with X > Dissolve Edges
Remove doubles with M > By Distance
Delete hidden or unnecessary geometry
Use the Decimate modifier on a duplicate first
Retopologize manually for cleaner results
Use Remesh only if a new topology is acceptable
Check shading after reducing topology
Recalculate normals with Shift+N if needed