Please, refer apoc.create.setRelProperty documentation
Attributes are Key Value pairs.
A small demo
data.csv
col1,col2,col3,col4,col5
T1,TA1,T1TA1,Medium,4
T2,TA2,T2TA2,High,5
T3,TA3,T3TA3,Low,3
T4,TA4,T4TA4,Moderate,2
T5,TA5,T5TA5,Very Low,1
USING PERIODIC COMMIT 100 load csv with headers from 'file:///data.csv' as row
with row where row.col1 is not null
merge (c1:Col1 {id: row.col1})
merge (c2:Col2 {id: row.col2})
merge (c1)-[r:HAS]->(c2)
with r,c1,c2,row
CALL apoc.create.setRelProperty(r, row.col4, row.col5)
YIELD rel
return c1,r,c2;
╒═══════════╤════════════════╤════════════╕
│"c1" │"r" │"c2" │
╞═══════════╪════════════════╪════════════╡
│{"id":"T1"}│{"Medium":"4"} │{"id":"TA1"}│
├───────────┼────────────────┼────────────┤
│{"id":"T2"}│{"High":"5"} │{"id":"TA2"}│
├───────────┼────────────────┼────────────┤
│{"id":"T3"}│{"Low":"3"} │{"id":"TA3"}│
├───────────┼────────────────┼────────────┤
│{"id":"T4"}│{"Moderate":"2"}│{"id":"TA4"}│
├───────────┼────────────────┼────────────┤
│{"id":"T5"}│{"Very Low":"1"}│{"id":"TA5"}│
└───────────┴────────────────┴────────────┘